In addition to other thoughts given...I noticed you said you ran a wet patch down before shooting...A wet patch will dampen the next load, even if you think you dryed thoroughly...Only dampen a patch before or between shots...

Other than than, make sure the flash channel on the drum and nipple are clear...If your gun doesn't let powder into the drum area easily, you might remove the drum and cut a taper on the inside, I have done this on several percussion guns...