How bout this Richard:
chamber 1. hot cap, Black MZ, ball.
chamber 2. hot cap, Olde Eynsford, ball.
chamber 3. hot cap, Olde Eynsford, over powder wad, ball.
Chamber 1. tells you whether or not it's the Black MZ alone. Chamber 2. tells you whether it's "any" powder. If 1, 2, and 3 all fire, it's Black MZ and the wad working together. I don't see how Black MZ can dribble down each nipple, penetrate the sealant over the priming powder, and kill the cap every time. A vapor though, sealed tightly in a chamber with wads and a lead ball would take the path of least resistance and go right to the cap. A liquid would do the same, but would have to saturate the powder before arriving at the primer, and then penetrate the primer, I'd expect that to kill the whole charge, which didn't happen. Final possibility is that the hot caps have a very limited shelf life, and you're the first to notice it and post about it. It could also be the number of cats, don't know how i missed that possibility. ???