Palehorse
Personally before I got to carried away with changing nipple or even caps, I would try a couple of other things first. Since you have cleaned and you have removed the cleanout screw and checked that out - My suggestion would be to pull the nipple out and even the clean out screw. Pour 10-20 grains of GOEX down the bore tilt the rifle to favor the powder getting out to the nipple area. Tap the side of the rifle and verify powder is coming out of the nipple and clean out areas.
If you are getting power to the those two spots - pour the powder back out push air through the breech plug with the bore facing down. Re-install the nipple and the clean-out screw. Take the rifle out in the garage or a safe area. Place a cap on the nipple and with the muzzle and inch or so off the floor - shoot the cap. The pressure of the cap going off should be seen on the floor especially if you have some dust/sawdust right under the muzzle. Also you could do the same thing at night or in the dark and you should see sparks out the muzzle.
If it does the above it really should ignite the powder, especially real BP, if it doesn't better look for some new powder.