Same thing on my CVA - I had a couple 'misfires' which were simply a case of the primer backing out of the breechplug, and me not noticing it. Now I wrap a little piece of scotch tape - prob -1-2mm wide - around primer before loading it; it stays nice and snug, and I keep a few similarly modified primers in my bag. I think maybe it'll have better fit once I've fired this gun more (only have a couple dozen rounds through it)

Killed doe with it the other day - 90gr Pyrodex & 240gr XTP. Shattered shoulder, DRT. Very satisfying.