Sounds like it might be an inadequate headspace issue. What length shells are you using (2 3/4", 3", 3.5") and what length is your barrels chamber (it should be stamped on the barrel)? The reason I ask is that if you are trying to chamber a 3" shell in a 2.75" chamber (or a 3.5" in a 3" chamber), it's possible that the loaded shell is actually contacting the forcing cone and effectively headspacing the shell on the mouth instead the rim. If this is the case, the bolt may not be locking forward all the way and causeing the gun to fail to fire (as it's designed to do) until that shell is repeatedly crammed into the forcing cone enough to fully close the bolt. If this is the case you have a potentially deadly problem if you continue to use the longer shells because pressures can easily spike to dangerously high levels. You are effectively creating a partial barrel obstruction because the crimp cannot fully open to allow the wad and shot to pass without excessive constriction.

I'd strongly advise you to not shoot the gun until you are sure that the shells are at or below the chamber length stamped on the barrel. If you did shoot shells that are longer than the chamber, I'd strongly suggest that you take the gun to a gunsmith and have it checked for signs of excessive pressure to ensure it's still safe to fire.

Mike