We'll need more info to help. Does it properly eject the spent round? Does the next round not feed from magazine or does it snag on the chamber or something else or simply not even pull a round from the magazine?
Typically SBE's like a diet of heavier loads before starting to shoot light ones. Yours is backwards. Is this a new or used gun? If used, I suppose it is possible they put a lighter weight spring in so that light loads (below 1 1/8 oz DRAM) would cycle and it now won't feed heavy loads. The typical issue is when shooting light loads, the gun will fire, and it will cycle back, eject empty, then close on an empty chamber. This is because initially there isn't enough force to release the shell stop and allow a round in the chamber.
Have you tried different brand ammo?