It sounds like the bolt or round is catching on something, the round isn't properly sliding into the chamber. Take it down and clean it completely then Check from the mags well inot the chamber carefully If you can find anything there try fireing a round id if it jams extract the round and inspect it carefully for and thing out of place, scratches, nicks, dents, in the bulet or case this may give you some idea as to where the problem is happening.