You know what gun you have, but we don't, as you don't mention it.
OK, more recently you let us know it is a percussion lock. That's a good start, but what gun is it?
To even identify what the cause of the problem is, one has to do things in an orderly fashion. I do it in this order:
1. Take the lock out of the gun and see if it is in working order. If not, then you are going to have to disassemble it to identify what is wrong and what you must fix. Here are a couple of links that will help on disassembly.
The Siler Lock assembly, at Muzzleloader Builder's Supply (muzzleloaderbuilderssupply.com)
This link shows a brass drift that has been filed to fit the square of the tumbler, so you can use it to remove the tumbler from the hammer.
Tips & Tools with Fred Stutzenberger – Part 12 : Black Powder 411
BTW, one of many things that could cause the problem with your gun is that the fly, that is supposed to go in the tumbler, was lost by someone and they put the lock back together without it. Here is a picture of a tumbler WITH a fly in it and the blue arrow is pointing to the fly. (This is a left hand lock, so the tumbler will probably look backwards compared to the one in your gun.)
View attachment 95387
2. Now, if the lock works correctly outside the gun, then you know either the lock parts and particularly the sear tail/lever could be binding on the stock or other problems. Coat the tail of the sear with lipstick, reinstall it and cycle it a few times. Look inside the lock mortise of the stock and see if it rubs lipstick onto the wood. If so, you have to clear/cut away a little wood where it rubs until it no longer rubs.
3. Since the gun has set triggers, the set triggers could have been inletted too close to the lock and thereby not giving the Sear enough room to operated correctly.
Gus