Have you taken out the trigger assembly, completely disassembled it and cleaned inside the hammer spring housing and the spring itself and made sure the pin that the hammer rotates on is clean.
Also if it is a very old gun that has been shot a lot the hammer spring may have sim0ply weakened and need replacing. Springs will weaken over time and continued use.
If i put a shell that has been already fired in it, it will fire everytime.
Uhh, I don't get this. How can a shell fire if it's aready been fired?
9 times out of 10 it's a very dirty gun.