A big variable has been identified. The shooter.
Do you have a friend, preferably experienced with Muzzleloaders, that can shoot it? This is the variable I would try to eliminate first.
In my experience shooting black powder is a lot different than center fire, trigger control, grip, follow through, it’s just a different thing.
If I shoot muzzleloader on Saturday and a service rifle match Sunday I can barely keep my shots on back stop, the reverse is also true for me.
It is less than two weeks to your hunting season, I hope you get it figured out and enjoy the hunt.