Some times crud builds in the cornors of the riflings;when I run into a problem like that there are two things I look for,one is ramrod wear at the muzzle the othere you check for by scrubing the bore with boiling water and citrus soap or lye soap then brush really good with a stainless steel bore brush then use a good lead and plastic remover. The other thing that comes to mind is that the ball and patch combination might not be tight enough. Lee