Most likely you're using a thin .015 patch.
With the traditions hawken ( I have built 6 of them and still have 3 for myself 4 if you count a factory made flintlock ) you're going to want to use at least an .018" patch with a .490 round ball. Check the tang screw every 10 shots as they often loosen up unless the tang and trigger assembly has been bedded. Try the swab the bore with a water based ( or straight water ) after each shot.
Don't bother weighing balls, its a waste of time. I shoot mine out to 200 yards and it doesnt care if a ball is +/- 4 to 6 grains.
While RS or Select is a good powder, I'd suggest switching to Pyrodex P if real black powder is not available. 70gr pyrodex P, .018 to .020 patch and a .490 round ball is a heck of a shooting load in all my traditions hawken rifles.
One thing that is highly important is consistent seating pressure. If you are getting sloppy and not seating the ball with somewhat fairly consistent seating pressure, thats going to show up, especially at longer distances.
Chances are, your rifle still has the plastic sights? If so, you need to cut a dovetail for a rear sight and get something real. Plastic sights have no business being on any gun.
90gr 2fg gearhart owen black powder with a .490 round ball and .020" patch at 100 yards. Thats a killer group for such a low cost muzzleloader.
These are highly important areas to bed on the CVA & Traditions sidelocks.
This you may find helpful as well,
https://buckskinsbp.blogspot.com/2019/10/choices-galore-substitute-black-powder.html