I really suspect it might be the nipple on the rifle. Is this the original Lyman Nipple? If it is, change it out to a stainless steel hot shot type. Second, when I hear such a report I wonder how the rifle was prepared to be shot. You are swabbing all thecrud and such out the barrel, and also popping three caps through the nipple prior to shoot to clean and clear the fire channel, and also carbon up the inside which helps the sparks bounce through it better.

When you think of it, put a patch on your jag. Push that to the bottom of the breech and pop a cap. Pull the patch and make sure there is a definite burn mark on that patch. If not, pop a cap through the nipple and do the same thing over again. When you see a definite burn mark on that patch, that means the fire is making it through the rifle nipple and bolster.

Next time you load the rifle, turn the lock, side downward and with the flat of your hand, slap the lock three times moderately. No need to hurt yourself mind you, but make sure that powder is falling under the bolster. Next take your nipple pick and pick the nipple. Some rifles never need to be picked and some demand to be picked. Make sure you are pushing through the nipple and just not hitting the bottom of the nipple with the pick. This makes sure that the fire will go through the nipple. Also it will make sure nothing from the cap before is blocking the nipple. Also use a quality cap. The hottest are the RWS in my opinion. Also the CCI magnum caps are good.

Try that and see if it does not make a difference. I shoot Goex FFg out of my Lyman all the time.