Sights and stocks can have a big impact on your shooting habits and how often you look into a scope or move your head to aim at an enemy.
If you are not able to line your barrel up in front of your nose then you are forced to adjust every time you shoot. This will cause you to attach your self to your sights, missing everything but your target.

heres some tips, next time you play move your eyes up down left right repeatedly and even when you find you target continue scanning. train your self by watching his movements out of the corner of your eyes and practice your snap shooting and firing posture.