As Cayugad said, make sure there are no cap fragments wedged up inside the hammer recess. That could cushion the hammer fall and cause a cap to fail to fire. Make sure that the flat in the top of your hammer recess is striking SQUARE all acrossthe faceof the nipple.

After several shots, it is possible for fouling to plug both the little hole in the bottom of the nipple, and also to plug up the entire flash channel through the bolster and into the chamber of the barrel. You might need to take the nipple out, and use a pipe cleaner or a similar stiff, flexible object to make sure the flash channel is clear all the way into the barrel.

If you can find them, get some aftermarket HOTSHOT type replacement nipples for your gun, and use a clean one every time you go to the range.

BTW, what kind of powder were you using? Pyrodex is relatively hard to ignite under any circumstances. I suggest that you use CCI #11 MAGNUM caps for ANY powder. A good, clean nipple and the Magnum type caps should resolve this problem.

Those Great Plains rifles are very nice, and I'm sure yours will work OK with a better nipple!