Do try a thicker patch. If the gun is a true 12 bore, with a .729" bore (or .730"), then the ball I'd use would be no smaller than .715", and that one with a .022" denim patch. I use a michrometer for measuring the cloth's thickness. I buy the cloth at a yard-good store (sewing stuff store), and choose only that which is 100% cotton and generally only denim.
; With LehighValley lube, my patches are WET, not damp, not pressed out, not licked damp, but WET. for hunting, I use a wad of cloth, disk of card stock, or an oxyoke wad to separate the powder from the wet patch. You don't want any lubricant migration to spoil the powder charge or vary ballistics from being wet.
: The muzzles of your rifle need to be radiused and smoothed where the angled crown meets the rifling. I've yet to see a productnion rifle with enough radius on the crown. For best loading of snug combinations, it should be not less than 1/8"deep, and 3/16"is better. That is, from the top of the muzzle to the full diameter of the lands should be 3/16", and from there, the angle is smoothed out to just a bit deeper than the bottom of the grooves.
: You should be able to shoot all day without having to wipe the bore. If you can't, you are #1/.- not using enough lube or #2/. - you are not using a thick enough patch or #3/. your crown is too sharp and is not allowing you to use a tight enough patch, with sufficient lube to keep the bore from crudding up.