I built a 5.56 AR15 from an 80% lower and a CMMG upper and CMMG lower build kit. About once a mag the bolt locks back with rounds still in the mag. It's not jammed. All I have to do it hit the bolt release and it chambers a round and shoots again just fine. It has happened with three types of 5.56 ammo and both steel and poly mags. What might I be missing?