IMHO, it can be hard to diagnose the patient when he is not in the office...
I would guess or suspect that your issue has less to do with your ability to clean away blackpower residue, but rather is a larger issue of two things:
1. Blackpowder fouling that has built up across the angles of the breechplug face and the face itself that not being removed has now been baked by all of the heat of charges to be concrete-like coke deposit. And has grown and thickened. (I have pulled breech plugs to find some guns kept "meticulously clean" by the owners with teh bore completely sealed off by a wall of coke.)
2. Your tools are not adequate for the problem or for preventing the problem. Putting a relatively "round" ended brush or cleaning jag into the breech tends to miss the 90 degree angles between the barrel wall and the breechplug face. Even a bore scraper is not 100% as in order to be small enough to fit inside of the bore, it still leaves a very gap at that 90 degree junction.
BUT, it certainly helps a great deal.
Many lads have spotless, bright bores everywhere except at the 90 degree junction that they cannot see until the breechplug comes out. (And some lads have older guns with no breechplug but rather a patent breech- and some of those have a cupped "chamber" at the breech that tools are too larger to into to.)
I would recommend considering what some lads call a "range rod" or a range "cleaning rod." These are like wooden ramrods with a brass ferrule threaded for NUG male tools to be added.
Many lads clean, in the field, with their metal ramrod and wiper. Then come home and do more thorough cleaning with the range rod and better tools like a scraper.
If you or a pard have taps and dies. You could use a piece of brass rod stock, and male and female its opposite ends to fit the metal ramrod and the scraper. I think it is just easy to have a wooden range rod for use at home.
In gleichem Schritt und Tritt, Curt Schmidt
Not a real Civil War reenactor, I only portray one on boards and fora.
I do not portray a Civil War soldier, I merely interpret one.