Yes, everybody here has pretty well diagnosed the problem. With a full 30 rounds, the magazine may be difficult to seat with the bolt closed.
I was taught to slap the bottom of the mag to seat it and have since moved to the push-pull method (push magazine in until it clicks, pull down to insure it is seated), though I still revert to slapping it on occasion.
My method now is probably more like push magazine in until it clicks, pull magazine to insure it is seated, if magazine comes out slap it back in and beat on bottom of the mag like it said something bad about mom.