I have had similar problems and it was due to the cap not sitting all the way down on the nipple. I would get a dent on the cap but it wouldn't go off. Try making sure the cap fits all the way down. Also, some different types of caps would fit the nipple and others wouldn't, so some experimentation may solve your problem. Musket caps are hotter, and may give you a better ignition, so using them is probably better even if you get your caps to work.