Here is a
film of a one shot kll Here is why it was a one shot kill...
A 20mm in the cockpit. So for one thing, a shot that kills the pilot will result in an instant victory.
Another issue you're probably experiencing is the lack of depth perception in the game. What may look like a nice tight shot group from 300 yards away, may not be as concentrated as it appeared. A number of times, I have gone back and reviewed films where I wondered why didn't that shot do critical damage? Upon viewing it in slow motion from a different camera angle, it's not uncommon to find that what looked like a nice shot group, were actually spread out across the plane quite a bit.
What is likely happening is that your hits are not concentrated enough on one plane part to do the critical damage. Small pieces of desbris are modeled to indicate some hits, but that does not always mean you've actually removed a critical component from the target plane.
Pretty much, each area of a plane can absorb X amount of damage before it will fail (eg. outer wing, wing root, aelirons, tail planes, ect.) The trick is to get enough hits in one area to cause a failure.
The only thing I can say is keep practicing, and approach fights so that you can set up shots that you are good at hitting. Noting what range you normally shoot at, and adjusting your convergence accordingly may also help.