Shoot the rifle, a lot.
I don't know why but just about every new flintlock I have fooled with tend to act up until it settles in. I would try different priming powder, picking etc etc.....full pan, half pan, flint size orientation, frizzen face condition...just shoot it. Most of the time they seem to work in and become less finicky.
So give it some time...learn the new set up....let the new rifle learn you.
No improvement after a few sessions of shooting, maybe you will need to further. First though I would put some ball down range.