Your problem is very simple! You are not getting the cap all the way down on the nipple. The hammer has to push the cap the rest of the way down and loses enough power so the cap will not fire. If you have trouble getting the caps seated all the way, try Remingtons.

Do not put the hammer on a empty nipple. That's what likely caused your "malfunctions" with both guns. Repeated dropping of hammer on empty nipple causes a flair in the tip round edge. Even when storing rifles, putting hammer on half-cock is the way to go. Full cock causes spring stress & spring enlargement over time... but not half-cock.

I had the same problem on my TC Renegade with a hot shot nipple, changing nipples solved my problem.