It is very strange that it is the SECOND shot after swabbing that won't go off. If cleaning/swabbing the bore was pushing fouling into the patent breech and blocking the flame channel, then why would the first shot go off after swabbing but then the second shot is the one that gives a fail to fire?
That is a tough one to wrap my brain around.
When you swab, how damp is that first patch you run down the bore? The "damp" swab should be only damp, absolutely nothing can be squeezed out of it.
I really have no answer for why the 2nd shot after swabbing is the problematic one???
I have a TC Renegade. They have shallow grooves and I've found mine cannot be shot without swabbing and keep good accuracy. Damp target type patch lube, yes, I can load and shoot all day without swabbing but the 4th shot after not swabbing, and all shots after the 4th, it will not hit a 2' x 2' box at 50 yards. No matter how tight of a patch I put around the ball to try to maintain the same bore consistency when loading, after that 4th shot it goes to parts unknown.
Damp or grease type patch lube, swabbed at least every other shot, and it'll keep shots in a 1" circle at 50 yards if I'm doing my part well.
I, too, found that swabbing gave me fail to fires from pushing fouling into the patent breech, but it was the very first shot after swabbing that wouldn't go off. Pull the nipple and trickle a bit of powder in, replace, and fire a cap and it would ignite the main charge.
I cured that problem two different ways. At first I just popped a cap after swabbing and that would clear the fouling out of the patent breech, next load would go off fine.
Being frugal, especially now that #11 caps are like hen's teeth around here, I read on here about a second way to solve the problem. Pushing fouling down when swabbing is from the jag and swab being too tight of a fit in a fouled bore. Chuck the jag into a cordless drill and spin it on a flat file to turn it down, until it fits with the swab so that they go down a fouled bore smoothly, but then the patch bunches and pulls the fouling OUT of the bore. Works like a charm. I run the damp patch all the way down, wait about 6 to 8 seconds for the fouling to soften a bit, then pull it back out of the bore. If at the range I often forego even running a second dry swab with no issues, but hunting I'll run the dry swab for sure.
I like how easy it is to load after swabbing so I just swab after every shot. I have not had a fail to fire since I turned the jag down. I do not have to pop a cap and double my cap usage.
I can't remember exact numbers, but when I turned the jag down for my .54 I can remember that the finished product was really close to the diameter of a store bought .52 cal jag. That is using a 2" round cleaning swab with it.