DennisR,
You may be trying to hit things too far out. I would try setting your convergence back the the defalut settings, 300-400 and work on getting in closer with a better position. Follow the enemy in sted of trying to lead and make a snap shot. When you get in range from behind, go for the kill.
Here is a good link on gunnery:
http://www.simhq.com/simhq3/sims/air_combat/gunsights/The auther, Andy Bush, frequents the Help and Training fourm. It's a good place to ask questions like this.
I also find shooting at off line drones are good practice to get the feel, range and deflection to make a kill. You will see when you make long shots say at 800 you will just damage them. When you get under 300 they explode with just a small burst of guns. When you make them explode, you have a better chance at getting the credit for the kill
I spend some time shooting drones every now and then when I feel something isn't right with my gunnery. It seems to help. Good luck.
Gunner