[htimsnuG]

You are probably not squared up to your target enough this puts the weapon at kore of an angle accross your chest putting the pec in the way and forcing the butt to purchase into the pocket of your bicep rather than in the shoulder.

Unless you are shooting some sort of long range in supported match when you need to get a mechanical lock of your elbow/twice of your off hand into the side of the body their is no reason to be situated like this because it drastically limits your field of view off the weapon and where you can swing the weapon to engage multiple targets buy simply rotating your waist , that last part is very important because it allows you to walk with the gun while firing

Adjust your butstock down so its short for you , stand faceing a target bring the gun up to firing position the stock should be short enough that when you bring you cheek down to look though the sites you should be able to almost take the thumb of your firing hand and bring it back to your nose.
Now that your stock is fitted to you stand in front of a mirrior , bring the gun up to firing position and see what you look like , tuck both of your elbows down toward your body narrowing your profile , now slightly bend your knees and lean forward at the hips , this makes your shorter. What you have now effectively done is
1)made a smaller target for the bad guy to shoot at
2)enabled yourself to walk forward
3) given your body a stronger way to absorb and mitigate the recoil
4) enabled yourself to make a fast and easy transition for strong side to weak side letting you cover 180+deg of field without having to move anything more than your waist.

Thier is a lot of different oppinions on where to place your weak hand , I personly do not like a forward vet grip (several reasons) so I pull my hand up against the mag well , this gives me great controle over the gun with less body movement and the hand is already used to being their which makes mag changes faster and more accurate.