In Episode 4:

• Rhaegal was flying in a straight line at a steady speed and too far away to represent an immediate threat to the men on the ships. This is a best case scenario for Euron's men being able to hit the dragon. Notice that once Dany and Drogon were aware of them and moving in a less regular pattern they could not hit them.




• The scene at the end with Missandei was under a flag of truce, Dany's token gesture of giving Cersei a chance to surrender without bloodshed. While very, very angry, Dany was smart enough to wait for the rest of the army to arrive.

In Episode 5:

When Dany and Drogon attack the Iron Fleet and King's Landing, they cannot be hit by the Scorpions. The reason they cannot be hit is that Qyburn's updated weapons are great naval weapons but absolutely terrible anti-aircraft weapons. Far worse than the smaller design that Bronn used to injure Drogon last season.

Bronn was able to aim and fire the weapon by himself. The newer designs are much stronger but they are too large for a single person to move while aiming.

On Euron's ships the person who is sighting and firing is sitting in a little chair at the back and is completely unable to actually aim the thing. There is a pair of crew-men at the rear moving a large handlebar around to try sighting it.

The scorpions on the walls are worse. They don't even have a guy in a chair to see how they are aiming. The guy in charge is standing in front and to the side while giving commands. They would be lucky to hit a slow moving boat.

It was not shown in the episode, but a realistic version of aiming at a dragon that is swooping, soaring, zigging, zagging, and filling the crews with a rational fear of dying would have the guy at the rear yelling "up, up, up, down, up, left, left, my left dammit, back to the right, right, right, right, down, fire, we missed, it's coming our way!, AAAAHHHHH!"

This episode and reminded me of a concept from the game Eve Online called turret tracking. It is (basically) a spaceship dog-fighting game.

The idea is that some guns can turn quick and others turn slow. If you are a small fast ship fighting a ship with slow turning guns the best action is to get very close and spin around it.

This is essentially what Dany did in this episode. She comes from above (presumably they cannot aim so high if you are so close -- combined with the element of surprise) then stays very close to the ships darting around.

Note how both the blue and red examples flew the same distance but because red is closer the scorpion has to turn a much greater angle. We see blue is maybe about 45 degrees while red is closer to 80 or so.

In the last fight involving scorpions (in addition to the surprise) the scorpions were very far from the dragons making tracking much easier*.

*: Of course, there's other reasons why hitting would have been difficult then and in both scenarios, but I am showing why tracking speed was not a concern then.

In the first instance, Dany, Drogon and Rhaegal are not in battle stance. They're just "voyaging" to dragonstone.

When Euron attacked Rhaegal, both the dragons and Dany were not ready to react. They were caught off guard. As to why Drogon and Dany didn't burn off Eurons fleet, Dany just lost one of her babies, and she probably wasn't even sure of what to do with all the scorpion spears coming towards her, she was surprised and unready and also distraught over losing one more dragon. It would have been probably foolhardy to go attack a prepared fleet with an unprepared plan.

In the second instance, Drogon was stationed far off because it wasn't a battle or an attack scenario. They were out there to negotiate Missandei's release and Cersei's surrender and not to fight. Again attacking without a plan would have been a wrong move, when attacking during the talks was not even on the card. Personally, it also felt like Drogon was kinda put out of reach incase Cersei tries to attack. Dany had just lost one dragon and wasn't about to lose another one without thinking through.

As to why Drogon and dany were more fierce in episode 5, is because they were in a battle. The dragons are fierce and seen in lots of instances before. Battle plans include how to get the fleet, the walls, the city and the current queen. She swooped in from the top, manoeuvred, knowing the ballistas on the ship cant turn and attack her in every angle she moves. She burned down the fleet destroying the morale of the fleet. Once the chaos ensued, she went on to destroy the walls as probably planned.

She was fierce, prepared and ready to attack in Ep5 whereas in episode 4, she wasn't prepared.

• Dany was initially un-experienced in dragon warfare, and it cost her her first two dragons. By the time she attacks king's landing, she understands the dangers and is prepared to deal with them.

This is demonstrated by how effortlessly she dodges the scorpion shots of the Iron Fleet, and destroys the scorpions on the ramparts before they can launch. (Presumably, with the ramparts, her angle of attack is such they they don't have time to re-orient the scorpions, and Euron's ship is destroyed because, after missing with the first few salvos, he can't get his ship turned around in time.

Because aerial combat tactics didn’t exist. In the first encounter, no one had had living memory of fighting a dragon. At the battle of the Gold Road, no dragon rider had ever encountered anti-dragon weaponry before, so there had never been occasion to evade. At the attack on Euron’s fleet, although people had used the sun for cover before, nobody had ever applied using the sun to cover an aerial dive bombing run.

All these tactics we understand now didn’t get their start until after the invention of the airplane.

Making it even more challenging is that the dragons weren’t the equivalent of starting out in Wright Flyers or Sopwith Camels. They were much more like modern A-10 Warthogs in maneuverability and weaponry.

Earlier in the episode it shows how Rhaegal was injured, with holes in his wings. Jon specifically says that he wouldn't ride him because he was just barely flying again.
As clearly shown, evasion of the scorpion bolts is completely with a dragons capabilities, however, Rhaegal was barely flying properly and such evasive maneuvers were impossible for him.

Drogon is more experienced.

There is a similar Question over on SFF.SE with a nicely summarized answer.

Drogon is the biggest
Drogon is essentially the big brother of the
three. He's the largest, potentially the fastest and most probably the
strongest; he's the alpha.

Drogon has Dany in the pilot seat
It would make a lot of difference
that Dany is the one riding Drogon, able to give commands, and steer
him better than the other two may.

Drogon has the most combat experience
Being Dany's main ride, Drogon
has experienced a lot of attacks, he has gained the most experience
under attack than his two siblings.

Viserion was hunted by the Night King
Olympic level javelin throwing
is an understatement for this guy. The Night King took down Viserion
as they were retreating with an epic throw of his spear. He's
powerful, that spear was travelling very fast, and hard. It pierced
Viserion exactly where it needed to to mortally wound him.

Rhaegal was injured already
In the scene where Rhaegal is shot down by
Euron's scorpion, we can see that Drogon and Rhaegal are flying in a
tight, low and slow formation. Rhaegal is visibly injured, his wings
torn from the Battle of the Long Night. This makes him slower, and a
much easier target.

Besides, Euron had the element of surprise; he could take his time
with his shots and make it count. Notice how once alerted, Drogon was
able to evade the rest of the arrows?

Drogon was in fact hit once Recall in the Seventh Season, Drogon is
hit by Bronn's scorpion, severely damaging his shoulder. So he's not
invincible.

---

_{Why is Drogon so much better in battle than Rhaegal and Viserion? - SFF.SE}

When Rhaegal is killed it seems apparent that Dany wasn't expecting the Scorpions in that magnitude or seabound and she was also already angry about losing one dragon, why risk the other?

In S8E05 Dany wasn't looking to intimidate she was looking to negociate (or at least Tyrion convinced her to).

I don't want to see this city burn. I don't want to hear the screams
of children burning alive. No, it is not a pleasant sound. I-- I don't
want to hear it. Help me save this city. My lord, I am only a
mouthpiece for our queen. TYRION: Your queen. Cersei is Queen of the
Seven Kingdoms. You are her subject. Her reign is over. You understand
this. Help her understand it.

---

_{Game Of Thrones, Season 8 Episode 4 (The Last of the Starks)}

A dragon pacing around would have put everybody on edge and probably decreased their chances in successfully negotiating a truce. Furthermore, if Drogon had been sitting in the front lines with Dany, and they had chosen to attack he would be an easy target for the dozens of scorpions. So, no strategic or diplomatic advantage would have come from Drogon sitting front and center.

In S8E05, Dany (and Drogon) knew what they were getting in to and were now preparing to evade the shots fired from the scorpions and knew not to come from in front but go straight down the middle and spend as much time as possible circling The Iron Fleet to make aiming as hard as possible.

So, no Drogons powers weren't downplayed he was simply lacking the preparation and experience up until S8E05.

Tactics also played a part.

In Episode 4 Danny was caught by surprise and may also been a victim of her own hubris. As already mentioned she was in "ferry mode" and was foolishly not on alert.

In Episode 5 Danny is initiating the conflict and has adjusted her tactics to counter the land and sea based scorpions. There was a big hint that something was being planned when Tyrion told Jamie not to worry about the Iron Fleet.

To start with Danny attacked by coming out of the sun, ie with the sun directly behind her. A dog fighting tactic that was use to great effect in our history in arial combat dating back to WW1. Coming out sun makes it much harder to accurately aim at the incoming target.

After the initial sweep on the Iron fleet she seemed to keep low, as close to the water as possible, then "popping up" for the attack run. While the scorpions were effective against the realtivley slow moving ships with the unsullied on board, tracking a fast moving dragon that is potentially below the actual level of the scorpion itself, is a much harder prospect. Keeping low and between the ships also increase the hazard of friendly fire for the Iron Fleet.

A similar tactic was employed against the land based scorpions. Again Danny kept low and close to the walls. The scorpions, both land and sea based, are mainly designed to aim up, dragons fly in the sky after all. By keeping low and against the wall she significantly reduced the effectiveness of the scorpions. Again she would pop up for the attack run, initially with a greater element of risk. With each attack run the risk was reduced until we see her basically attacking with impunity along lines of scorpions that were pointing out from the wall. Once the anti air defenses are taken out she is free to attack at will.

In short, the Lanister forces did not really adjust their tactics as they had previously worked, and failed to account that the opposing forces would adjust theirs.

The only reason, is because it was written that way. Those weapons would have been lucky to hit the ships, much less a dragon. Nobody firing a newly developed weapon, with no sight and no experience firing at a moving flying creature (from a boat) would hit, much less multiple consecutive hits.

You need to be able to accurately determine the speed the dragon is flying and the distance to the dragon. I believe this would amount to shooting at a spot 200-400 yards in front and 100 yards above the flight path of the dragon. Typically you would have used trial and error to figure out where to generally aim. I would think they would have had a much better chance of hitting up close.

This blog covers the build of a modern ballista. This page shows 13 foot groupings at 790 yards on a stable platform with a sight. Not going to happen on a ship, with a sightless ballista.