Call Mossberg and tell them your problem. They may have a clue why the gun is doing that. I have no idea, can't see why it would make a difference unfired vs fired shell, etc.

Is it a new gun? If it is an older one, there may be worn parts, or it could be dirt inside the bolt or around the firing pin spring.

Tear it down completely, clean all parts and inspect for broken or worn parts, the reassemble. If it still acts up, I wouldn't use it until Mossberg or a gunsmith can diagnose and correct the problem.