Trying to diagnose an issue with my new AO M1 carbine but can't seem to find the culprit. Shot it for the first time today and the first few mags went off without a hitch. Was shooting Tulammo. The first box gave me no trouble at all, but part way into the second box, after firing a round, the bolt would only travel maybe a quarter inch back and lock up there (though occasionally it would almost go back far enough to eject the spent casing.) Manually cycling the rounds seems to work fine, and the gas piston seems to move freely. Took the rifle apart but couldn't find anything amiss. Any insights?