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.
Black Powder: Not because it is easy, but because it is hard, because that goal will serve to organize and measure the best of our energies and skills, because that challenge is one that we are willing to accept, one we are unwilling to postpone, and one which we intend to win...