Quote Originally Posted by Dieudonne' Saive. View Post
It's like the bottom of the bolt, is not grabbing the case head a it passes. Multiple slow, deliberate passes of the bolt will not pick up the tail of the round at all. This is different than the round being picked up, but not chambering.
remove the magazine, re-insert, seat the magazine with a good hard slap, and try again.

Sometimes the magazine don't fully seat (even though it clicks in and you cannot make it move deeper).