Do the back end of the firing pins look like they may have been shortened?
They sometimes mushroom at the back end and a well meaning fix is to grind or file the damage off. That makes them sleek looking again, but too short sometimes for good ignition.
They still protrude the same from the breech,,just short at the back end.
Another problem may be the lock(s) pulled overly tight into their inlets.
Over time and use, people tighten that lock plate screw ever tighter. Pulling the plates and the mechanisms into the wood inlets inside.
It doesn't take much to slow the hammer down. Rubbing against wood or actually bending the plate.
Even an old repair to the wood inside the lock cavity may be sitting a bit high yet and interfering with a hammer or spring. They crack a lot up at the front and repairs done of different skills are often in there. Some helpfull,,some not.
Check the face of the hammers where they contact the firing pins and make sure that surface is square to the back of the firing pin. Hitting the firing pin at a glancing angle can cause it to loose a lot of energy.
None of the parts in those are really a masterpiece of fitting and craftsmanship, doesn't matter what grade,,,so it doesn't take much to start a problem.
