Well, I've hunted muzzleloader long enough to make about every mistake, so I'll take a crack at helping.

I assume this is an inline rifle? Either way, musket caps ought to be plenty to make it go boom every time.

SO, the troubleshooter in me says there are limited possibilities - only three I can think of:
1. The cap is going off each time? Easy to check and you must know. I'll assume the cap is popping....
2. Is there moisture or oil in or near the breach? Best way to do this is to ALWAYS fire off a couple of caps before beginning to load. I know some people who don't believe this.... most of them have missed good animals.... :-) I won't hunt any other way than with a cap-dried barrel.
3. Possible poor spark path? Best way I know of to check this is to clean the gun thoroughly and then put a clean patch on your cleaning jag, run it all the way down, leave it all the way down and and fire off one cap. One musket cap should be plenty to really blacken the exposed part of the patch. I mean really blacken.

I'm guessing spark path. If it's an inline, you should be able to remove the hammer and peer thru the hole straight into a light source to see that you've got a good straight path.

Good luck. Let me know if you ever figure it out, whether I was on the right path or not.

Thanks,