Some things to try: If you are using low brass bargain ammo, it might not have enough power to cycle. If the gas system is adjustable check that, read the manual. It might be set for a more powerful round. Drop in some buckshot and see if it works for that. Also might check that the gas system is assembled properly and clean. Finally, check the extractor to see it it's there and functioning.
hard to tell without seeing exactly whats happening. Is the shell moving partially out of the chamber? Not moving at all? Double feeding?