I had a similar problem with a Stag I just purchased, The mag that came with it was the problem, though. However, after comparing it to the Colt, the bolt catch did not move quite as freely. I took it out, stoned each side of it and snipped off 1/2 of a coil. Cleaned it, lubed it and put it back on the lower. Now everything is fine. . . except the mag. Sometimes the spring on the catch is stronger than the mag spring and the follower cannot push it up into position to lock the bolt.