1) are you swabbing the bore between shots?
If you are and still having problems, the guys here in this thread had some good solutions.
Here's more suggestions.
T/C also makes their pre-soaked patches in "T-17" (the blue ones). They are formulated w/lube to make loading easier. Be sure to roll the wet-patch into a ball and squeeze out the extra solvent so it's not too wet going down the barrel. This can (obviously) foul your breech plug. Another method I've used is to spray a patch lightly with Ballistol and run it down before seating the sabot.
T/C's like a tight sabot! If you go with something smaller, you will lose accuracy. It's a trade off.
“A man who opens a case in hunting camp and lifts out a single-shot BP rifle makes a statement about himself. It may be quite unconscious, or it may be pure egotistical one-upmanship, but it does project the image of a superior marksman and a hunter of above-average skills.”