Xxl, first thought; try a different magazine. It sounds as if there is a small difference in the magazine position from where it should be to lock back, and to strip a new round out of the mag when it is locked back. Need to rule out a possibly defective magazine. Also possibly it could be a tolerance issue between the upper and lower. Any visible gaps between them? Also, switch ammo to make sure it is not a specific issue with that load. With that I will sit back and let the really knowledgeable guys weigh in.