This year prior to departing camp and going home I attempted unload my gun. I went through five musket caps before I had a cap pop and fire my gun. These musket caps were bought just days before the season started. There wasn't even any wet weather this year. You can probably imagine the thoughts going through my head at that point. As I do hunt in Washington state, I've had to make my rifle western legal. That being said I have the western conversion kit on my rifle. In fact, I've bought two from Knight. The newer one has a different looking bolt. That is, that it covers the opening of the cap/breach plug more and has a little opening that still makes it western legal. With both bolts and breach plugs I've had the same issues.
I believe your musket nipple is treaded into the breech plug?
Put the nipple in with your hand, then back the nipple out 1 full turn. Then insert the breech plug (with the nipple out 1 turn), then put a cap on and fire.
I hope you get the picture?
This might tell you if the firing pin to end of nipple is the issue (head space - so to speak).
Thanks for the advice!