Careful with pressing hard on percussion caps to make them fit. I know of a shooter who blew the tip of his thumb off with an accidental discharge. (Left a nice cleavage scar for him to tell his grandkids about...) A lot of shooters here have great tricks to get around this. I always gently squeeze the outer rim of a No.11 cap and fit it on with no more force than I use to press on a keyboard. I do this on 2nd gen Navies and Dragoons. They almost never fall off in my experience.
If your cap is not fully seated, the "crumple zone" on the sides of the caps will absorb the hammer's impact causing a mis-fire. Another cause is that you may have crushed caps and fragments deep down in your gun's mechanics. Your hammer will fall, (click), but will lack the force to explode the cap. (Been there, done that.)
Solution: Take your gun apart and clean it thouroughly, clear any fragments & debris and then make sure that you are properly seating your caps. It worked for me so I hope it is helpful to you.
-Dave