Newtothis: I had a similar situation with my flintlock and it is not even a year old. That is the hammer would not stay back, I had to rock it back and forth a bit to get it locked.

The solution was what Cayugad suggests. The parts in my lock were getting covered in crud, preventing the lock from working properly. Take that lock out, its easier than you think, and give it a good cleaning. That should hopefully fix it for you.

~Robert