Everything that you hit did go down. Where you hit them, with what type of round and how fast they went down are the only questions that I could gather from the film. If you notice, you used up all of your cannon on the first P38, and actually finished that plane with machine gun rounds only. I don't have the answer for you why AH planes explode, but my best guess would be either the pilot was killed, or the cumulative amount of lbs of ammo has been delivered to that plane.
Let me look into your first concern your aim. Since you were hitting your target your aim was good.
Sometimes convergence is the issue, although your hits seem to be tightly grouped, you may have been closer than where they were set to converge, producing less damage to a particular structure. This is not to say that you did not throw a good amount of cannon rounds to the wind. This goes to my first impression that a number of hits were from machine gun rounds (which definitely need more hits in the same proximity to cause damage to structures besides holes).
Planes in AH have certain structures where hits produce immediate damage. Fast forward to about 4:30 or so and slow speed down and zoom in on the P38. P38 design places its twin engines out along the twin fuselages. Your rounds were striking the cockpit area as well as other areas, but I saw no evidence of engine, coolant or fuel damage until the kill shot was delivered on the right engine. Without wounding the pilot, or destroying a critical flight structure the plane will continue to be able to fly, even missing parts.
My suggestions would be:
1 Turn tracers on
2 Fire machine guns to acquire the target
3 Fire cannons (alone to be certain of which round hits) after target is acquired
4 Check range and convergence for your plane to maximize damage
5 Choose ammo load that fits your style of shooting (quick short bursts of cannon or spray and pray)
Lastly, I don't know if the 30 mm cannon was used in this flight, but those often do produce one or two hit kills that you are seeking, but at a price of limited amount of rounds.
