I have the same problem when drawing from appendix doing reload drills with 2 or 3 rounds in each mag. (Assuming you're right handed) More than likely it's your dominant hand thumb on/over the slide release, not allowing it to be pushed into place by the magazine follower. Normal shooting its not a problem for me but when I'm doing quick drills, drawing from appendix, my thumb usually ends up right on top of the slide release. When you're gripping the gun, try moving your dominant thumb over onto your support hand more and not as tight against the gun. Also, as others have said, the extended slide release is a great work around for this. I actually have one on the way for my carry gun.