Sometimes it takes up to three caps before they show a burn mark. Especially with a gun like yours. Just push a clean white patch down there, leave it, then pop a cap. Pull it. No burn mark, do it again and again until you get a burn mark on that patch.
Something else you might want to do when you are on a range. Take the nipple out. Then pack some powder into the bolster/nipple hole. With your little nipple pick (the thin wire in the nipple wrench) pack that down there good. Then put the nipple back in, cap it, point the muzzle at a leaf or something you will see move, and pop that cap. It should blow the bolster clean for you.
Something to remember should you ever (and you will) dry ball a load. That little bit of powder you pack in there, will fling a round ball 25 yards, making it hit the target and bounce half way back. Something to remember when you forget the powder charge. I am speaking from experience.
Also if your hunting and you want to be positive that the rifle will go off. When you have nothing to do, pull the nipple, put a little powder down there, and put the nipple back. Of course you realize when you are fooling around the buck will walk out. BUT with that little bit of powder in there, when the cap ignites it throws a massive flame down the bolster into the main charge which ignites it right NOW!