Check the fit of the cap on the uncharged barrel, press it on with your finger/thumb as normal, lower the hammer on to the cap slowly and press the hammer firmly on to the cap, if it seats a bit farther, you need different caps or a different nipple so the fit is not so tight. What's happening is the hammer seats the cap on the first hammer drop, then it ignites on the second hammer drop. I suppose you can also do this just prior to shooting, but I don't care for caps that fit that tight, but it does keep em from falling off which is just as bad, makes it real hard to deprime after the hunt.
Tim