Cqps are not all alike. I've used #10 caps on those guns - as long as it fits on the nipple and goes down all the way it will be fine. A larger cap can be pinched a bit to keep it from falling off. Since we cannot see the top of the nipple, I would not be surprised to find the nipple has been peened-over by dry firing. The top of the nipple should be flat and even, not mashed over with only a smallish hole left for the fire to get through.

Dry firing is the bane of percussion cap guns. NEVER do it.