Your getting bullet bounce:
I would try shooting and being very careful not to push up on the magazine from the bottom. This changes the leading edge of the magazine and can cause what your talking about. Also occurs during rapid fire.
It is possible that you have a weak main spring, and that the toggle is rebounding to fast and when the tip of the bullet hits the ramp it bounces up and hangs, but because it only occurs now and then my bet is on your shooting position.
It is also possible that the magazine latch has become worn, this will hold the leading edge lower than normal, so instead of the bullet sliding along the ramp, it hits nose first and then bounces up on its way to the chamber.
You don't have a big problem, it may take trying a few different things to find exactly what is causing the problem. Oh, and of course that means you'll have to shoot it some more
Vern