Russian BMM:
If you haven't removed the lock before, it's pretty simple.
Place the lock at the half cock position. Unscrew the lock bolt on the left side about 3 turns.
Lightly tap the screw head with the screw driver handle. A few raps should dislodge the lock from the stock. Back out the screw another few turns and repeat. Then remove the screw and the lock will easily be removed form the stock.
While your looking at the inside of the lock, pull the hammer back to the full cock position.
The piece that is sticking out perpendicular to the lock is the sear release arm.
Holding the hammer with one hand, gently push up on the sear release arm. This will release the hammer.
Slowly let the hammer down and notice if the little end of the sear arm that is on the tumbler goes into a notch in the tumbler with the hammer about half way down. If the sear engages the notch, the fly is either missing or is stuck and that is the source of your problem.
If the fly keeps the sear from engaging the half cock notch, it is working fine and the hammer should fall all the way down to the fired position.
If the hammer does fall like it is supposed to, the problem is probably one envolving something hitting the wood which keeps the parts from moving freely.