It could be any of several things. I believe that rifle has a patent
breech, in other words there is a chamber in the breech plug for the
powder, Running a patch down the bore still doesn't clean out this
chamber, since it is of smaller diameter than the bore. Your
cleaning jag will bottom out against the face of the breech plug.
You can check this by running the ramrod down until it stops, marking
it at the muzzle of the rifle, then laying it alongside the barrel
with the mark at the muzzle. The end of the rod will probably not
reach the bolster (drum) which is screwed into the breech plug.

Try using real black powder, if you can get it. You might also use a
nipple pick to make sure the flash channel is clear when you load. A
pipe cleaner might be better if you can work it through the nipple
and down the flash channel into the barrel.

Try using a .35 or .44 caliber brush to get into the patent breech,
whichever will fit.

Lastly, don't forget that in a hunting situation, you aren't going to
shoot ten times at a deer. If you are having fouling problems show
up on the last shot, there shouldn't be any problems with ignition on
that first one at the deer!

Hope this helps. I'm sure other folks will have their ideas to help
you.

-Kees-