Check for wear on the hammer assembly, it might be catching the edge of the cap and not striking properly, or the hammer might be worn.

I had a percussion rifle that started to misfire now and again. I blamed it on the caps being damp.
When I took a close look at the hammer there was a small amount of play on the shaft which allowed the hammer to move backwards when it struck and the inside edge of the hammer caught the edge of the cap.
I ground a few thou. off the inside of the hammer and it worked fine after that.

I've not seen Tac on her for a while. I hope he has not gone to the big gun club in the sky!