Clark;
A couple of things might help your problem. First smoothbore fowlers, muskets, and trade guns, rarely shot a patched ball in the past. So, either the ball was dropped right on top of the powder, and held in with a piece of old blanket, or paper cartridges were rolled up, and used. This is why modern smoothbore shooters are always struggling with the patch ball loads.
Secondly, vegetable oil tends to polymerize when subjected to intense heat, like in a smoothbore barrel. Think about it, what do you use to season cast iron cookware? Tallow will take a lot more heat before it starts to build a hard coating on the barrels interior. Venison, or mutton tallow work well, while bear lard is the best, it’s a little hard for most of us to get. Good luck.
Hungry Horse