You can use a jag end to seat the ball. That is all that I have ever used. Never had a problem.
You might try - just to see - putting a thin cardboard/fiber wad down the barrel after the powder is in and before you seat the PRB.
Ditto the advice about thicker patches. I have the GPR as a .50 and 0.010 patches give fine accuracy out to 100 yards. I do have a .54 barrel but it is a smoothbore; PRBs in that get a tight patch but since there are no lands or groves to fill, the thickness is not what you'd expect, usually a 0.010" (again) and a .530 RB (though I am always working on it.)
Pete