I was having problems when I switched to 209s also. Then I tried musket caps in both my T/C Black Diamond and my Knight Bighorn and Voila!, all was well again. I think the 209s, even Remington's muzzleloader 209s, produce too much flame to get through the nipple's orifice and into the breechplug. I always had a lot of blowback. Musket caps seem to produce just the right amount of flame for loose 777 or Pyrodex pellets. Yesterday, I shot the Bighorn with 110 grs. volume measured FFFg 777, 250 gr. Shock Wave and a CCI musket cap and they shot into less than 1 1/2". Switching to the 209s, the groups opened up to close to 3" and there was much more fouling in the action of the gun. I swab with a wet and dry patch between each shot.
Hope this helps,
Mantihunter