In my Black Diamond XR I shoot only 209 primers even though I have the #11 and the musket nipples for it. It never gives me any problems.

It sounds like the cap is setting itself. I have one rifle that does the same thing. It is one of my Wolverines. And this is what I do... (do this at your own risk)

1. the rifle is empty and I am ready to load.
2. cap the #11 and then and fire it. 90% of the time the cap will not go off, but instead sets it self. It will fire the next time I **** and fire the rifle.
3. Load the rifle
4. pull back the hammer and take the shot.

Somethings I noticed about my Wolverine. When it starts to act up... it usually happens when the nipple is fouled real bad. So I take a Q-tip and keep that nipple clean. A little alcohol on that Q-tip and rub the fouling off. Then it will normally fire the cap the first time.