It's very easy to leave a little crud on the breech face, particularly if your jag is cupped to fit the ball. That's the reason I make all my cleaning jags with a flat face. That helps a lot but it can still leave a little residue. After cleaning, if the shooter will lower the rod down the bore (without a patch) and let it drop the last couple of inches, he can hear a ping as the jag bumps the breechplug. That is a good sign you have a clean breech. If not, you can be sure there is fouling down there and may need to scrape it.
If one can remove their patent breech and measure the diameter as well as see what the profile is in the bottom, you could make or have made a custon scraper to fit. A friend of mine bought a patent breech for his chunk gun, but didn't want a chamber, so the breech maker drilled it out to bore size. He still had the profile of the drill used in his breech and he had me to make a convex faced jag that would clean all the way to the bottom. Works great to hear him tell it. There's more than one way to skin a possum!