I would try firing the same cap again (after 40 seconds) some times I have had this, the first hit knocks the cap on the second hit fires it, this happens if I have bought the wrong caps/some one has given me some that's the wrong size, a friend had a revolver with on nipple fractionally bigger than the other 5, so 5 would go off first time the last required 2 hits to set it off.
I bought a nipple primer for the times when a charge has failed to go off (usaly in some one else's gun) it has helped to get the ball out of the barrel without having to resort to the ball puller and the grief it involves (and in my case removing the oats filler/wad which when compressed between the ball and powder sets like concrete) the nipple primer fills the nipple with fine powder so when the cap goes off it hits powder right away and provides a bigger and hotter flame to set the charge off.