There are a couple of things to check -
First - obviously - you want to take a look at your nipple - how worn is it? If that is O.K.
Second - have you or do you romeove the lock when you clean it? If so, you may have overtightened the lock bolts that go through the stock and into the lockplate. Sounds very minor and it really is but I have had that problem on a number of rifles. The problem may be that with the lock bolts overtightened, the internal parts are tightened into the lock mortise and there is some interference somewhere. A way to check this is by applying a light coat of inletting black or even heavier grease to the inner workings and seeing if any transfers to the wood anywhere in the lock mortise - if so, you may need to "relieve" the wood to prevent it. Believe it or not - even if you haven't overtightened the lock bolts - a change in humidity may have affected it enough since wood does swell.
With an empty barrel pointed in a safe direction - place a cap on the nipple, cock it and pull the trigger - watch the hammer as it falls and see if it is hanging up anywhere along the travel as it drops on the nipple.
I'm not familiar with your gun - does it have a drum and nipple or a patent breech? If it is drum and nipple, check the alignment of the nipple and the hammer cup. Once in a while a drum will loosen and turn just enough to throw the alignment off even though the hammer cup encases the entire nipple (which it should) when it falls.
I don't know if I've explained this very well - if your gun has been working fine right along - then I'd check for binding in the mortise that is just enough to interupt/bind the hammer fall. Good luck and let us know what you come up with.
