0.01 patch is OK but what is the size of your round ball ?
It's possible that the rifling at the muzzle is too sharp, sometime that is sufficient and when the round ball is pushed the little hole that get started become big...
I remember on a Hawken that on this one I had prowling with a steel ball of truck wheel bearing and a wooden board to soften the angles. After that no more problem...
Look at the mouth of your barrel you don't spend money and the solution can be there.
Other way the rifling can be too sharp in the whole length and then you risk to be obligated to polishing a little bit this, if when you push a cotton ball inside the parts (bristle is it right ?) stays hooked, simple polish for metal on patches can be good enough...