If you are getting ignition on the second hammer strike consistently, that tells me the caps aren't seating fully when you put them on. It could well be as Low Key said, the angle being wrong would require a first blow to form the cap to the nipple so the second blow can ignite it. It can also happen if the nipples are battered, the flared tips keeping the cap from fully seating. Much less likely is a weak hammer spring.
At any rate, these pistols give nothing up to modern guns in any area except that they are really, really, really dirty and fragrant. If you aren't having reliable ignition, it's not normal and is fixable.