I was having the exact same problem with my .54 cal Pedersoli Mortimer. The first day I shot it, I managed to get off four rounds before I ran into problems. After that it turned into a one-shot wonder, and my ball-puller became dangerously worn :cursing: . This is what I do now with every shot:
a) Almost no lube on my patches
b) I don't clean the bore between shots
c) Wipe frizzen and ESPECIALLY the flint
d) After I load the powder and ball, I tap the
butt of the rifle on the ground 3 or 4 times
to settle the powder right into breech and I
shove down hard on the ramrod to pack
everything in tightly
e) If the gun fails to fire after 2 or 3 tries, I
will double the priming charge
Since I started doing these things, my rifle has become much more reliable, and it fires on the first try almost every time, and always goes off by the third try. I noticed fouling decreased significantly when I decreased the amount of lube I used on my patches. Also, I took my breechplug out one day and removed some burrs left over from the manufacturing process that were very likely blocking the main charge powder from reaching the vent area. I shoot Goex 2F and 3F and have not noticed much of a difference between the two, although I have noticed that atmospheric humidity makes a big difference in the reliability of my rifle. If I go out after a rain or on a particularly muggy day, the 4F priming powder I use cakes up really badly and I have a much harder time getting it to ignite. Right now I am shooting 7" groups at 30 yards, but that may be because I am still brand new to marksmanship, having only fired a gun perhaps 20 times so far in my entire life. For what it's worth, my rifle seems to be getting more accurate the more I shoot it.