Ahhhhh you will have a love/hate relationship with that rifle!! We have 2 of them in our family and they do the same damn thing. After you dump the powder down the barrel turn the gun over to the lock side and smack the side of the stock with the palm of your hand to shift the powder under the nipple.

these 50cals are a PITA for some reason. My .45 hawken gives me no trouble at all.

If its loaded now, just remove the nipple and dribble some powder into there. Its possible that you did not pump water through the breech area when you cleaned it and there is oil down there that gummed up the powder, therefore, not allowing the powder to shift under the nipple.