….. Put a new factory nipple on the gun. Clean the gun, with alcohol before shooting if necessary to get oil out of the flash channel. Snap two or three caps before loading. Use real black powder. After pouring in the powder, use your palm and smack the gun sharply two or three times on the side opposite the lock to settle powder into the flash channel. Then load the projectile and seat it just until you feel the powder crunch. Throw the fake powder away. Make sure your clean out screw is not so long that it blocks the under side of the nipple. …, Make sure you are not over torquing the lock into the mortise and causing internal friction to slow the hammer fall or that the hammer is rubbing on the stock to slow the hammer. Lastly, did anyone ever remove the drum on your gun? If so, the likelihood that they got it lined up correctly is pretty slim. If it is not exactly in line with the hammer face, the hammer will not hit the nipple squarely and it may take more than one strike to get ignition.
^^^^Another Vote for The Above ^^^^^^
LD