My TC Renegade has a patent breech and I used to struggle with fail-to-fire's (FTF) quite often when swabbing between shots. When swabbing I was pushing fouling down into the patent breech. A cap simply couldn't burn through it after I held the benign fouling in place with some powder compressed with a PRB, I had to pull the nipple and trickle some powder in to get it to go off when it happened.
I made a post here about it and a few different people offered the below information:
There are three answers to this problem, assuming you are using a swab patch that is only damp and not wet.
1. Quit swabbing.
Pour in the next powder charge, don't have your head over the muzzle in case there's a hot ember that makes it torch off, and then load the next PRB. The fouling is scrapped down on top of the powder charge and gets shot out. (I've tried a ton of different combos and still can't get my rifle to stay within a soccer ball for accuracy at 50 yards if I don't swab, so I don't use this method).
2. Pop a cap after swabbing.
Point the muzzle at a blade of grass, leaf, or even a dry patch you threw on the ground and make sure you see it move when the cap goes off. This indicates the flame channel and patent breech are clear. Load as normal. (I've done this here and there, it works fine but I'm frugal and don't like to use two caps every shot).
3. Turn down your jag a bit.
Chuck it in a drill and spin it on a flat file, taper a bit with the front being smaller. Check fit on a fouled barrel as you go. You want your jag/patch combo to go down the barrel smoothly, but then the swab patch bunches up and pulls the fouling back out of the barrel. (My favorite method).
Since turning down my jag I have not had a FTF to date.