In reading your report three possible problems come to mind.
First, you bullet isn't carrying enough lubricant. A better designed bullet might help. Also a card wad seated on the powder charge, followed by a lube soaked felt wad, then store the loaded ammunition nose down until ready to shoot.
Second, you may need a better lubricant. To eliminate guess work, use something that is tried and proven - like SPG - until you sort out the fouling problem.
Third, you didn't say what alloy your bullets are cast from. If it's too hard and doesn't upset in the forcing cone it can lead to all sorts of problems. Most BP rifle shooters recommend no harder than 20-1, lead to tin.
No guarantees any of this will solve the problems your having but it's somewhere to start.
Good luck,
Dave