You are right, Jim - I was way too vague. What is happening is I can cock the gun with the hammer in single action mode, and everything is fine; the cylinder rotates and locks up, the hammer stays back, the trigger moves back, etc. But when I pull the trigger to fire the gun, the hammer doesn't fall, it just stays back in the fully cocked position. The trigger moves back far enough to release the sear but the hammer just doesn't move. If you keep the trigger held back and push the hammer forward, it will fall, but takes quite a bit of pressure to do so.
The same thing also happens occasionally in double action mode but not as often. Maybe this will give some insight into the problem. Thanks so much once again.