The Pyrodex is the problem especially if you are using the R granulation. The combination of the chambered breech, all the twists from the nipple seat and the modest heat from the CCI#11 cap and stock nipple really puts a hamper on reliable ignition. You need lots of heat to ignite Pyrodex P. Pyrodex R is slightly easier to ignite. You will gain reliability with a nipple designed to send more heat from the nipple seat to the powder chamber. Red Hot, Spitfire or Hot Shot nipples will work better. CCI or Winchester Magnum caps or RWS Plus Caps send more heat along the flash channel. Quite obviously real black powder is the answer, but only if you can obtain some. Local suppliers may not be able to meet the storage requirements and are limited to the substitute powders. Apparently mail order is not something you care to do.
As others have said, make sure that the breech is cleaned of residual oils by using a dryer such as rubbing alcohol or acetone. Store the rifle muzzle down after cleaning to drain excess oil from the breech.