Hi. I have a Ithaca Nitro Special 12 ga vintage 1924. Gun is in very nice condition. I have a problem opening the gun after taking the shots. Not every time, but often. It is the locking lever that is very hard to move, nothing to do with firing pins. When the gun is empty, or has not been fired yet, everything is free and smooth. I have cleaned and regreased the barrel extensions where the locking ramp engages. What else might be wrong?