Bolt locking to the rear question

[ODA CDR (RET)]

I recently purchased a Rock River Elite Operator 2 AR-15. I took it to the range to zero and afterward did some range fire. After the third 30 round magazine, the bolt would sometimes lock to the rear as if it were an empty magazine (not empty though). This would happen three to four times with the next two 30 round magazines. I used the new RRA magazine and a new Magpul magazine, with the same results. The ammo is Federal 62 gr 5.56. In 30 years of firing M16 and M4's, I have never had this occur. I checked to make sure the bolt gas rings were not lined up and other than that, no mechanical evidence of something wrong. What could be causing this to happen and how can I correct it. Thanks