I really don't pay attention to anything inside the plane....while fighting or flying.
but I watched them a lot and my rounds, I would say fall behind the enemy more then anything else...
For the first.
All of the years I've spent hunting down gunsights and gunnery manuals to the end result of my last historic pack. Along the way I learned how to use WW2 gunsights the same way WW2 pilots learned how. That's why the AAF\RAF formula for a 100mph ring works in the game for me. You may remember some pictures I posted with lead rings calculated to a 300mph con for 30mm, 20mm and 50cal. Seeing main ring radii as 100mph settings is now second nature. From repetition those calculations are muscle memory.
I suspect you use the dot of death.
For the second.
If it's any kind of deflection shot between 5-30 degrees. Not enough lead and elevation accounting for the cons speed or realizing you have too much G in your turn instantaneous or sustained. Dead 6, you may be shooting from too far out or your con is diving or climbing a bit during the tail chase.
Here is where I wish Hitech would revamp the LCG reticle with a 100mph motion ring and fixed center along with speeding up the offline drones to 300-350mph. In the TA you would get a better sight picture and visual feed back from your tracers while chasing a real person.
I always though you had your Ki-84 well dialed in sub 300yd from my losses to you perspective. Unfortunately the things I covered above are from the perspective of a fixed reference called a 100mph ring. Same reference real Ki-84 and 109 drivers used in WW2.