I was having a problem with my caps not seating down far enough on the nipple for them to pop on the first hammer strike but thanks to some suggestions from here I filed the nipple ever so slightly with the finest file i could get hands on and it works perfectly now. Some guys also suggested just getting another nipple, which I had a spare to try but I thought I'd give the filing a shot first.

That might not be your problem, but I figure it's worth a mention.