It sounds like alcohol has helped a lot, but here's what I think is happening.
When you're swabbing your patch and jag combo is too tight, so you are pushing fouling DOWN the bore and blocking the flame channel. I fought this on my sidelock too. There are a few options, I'll talk about three of the main ones.
Many folks just snap a cap after swabbing to blow out any fouling that is in the way. Hold the barrel down next to something like a blade of grass, or someone mentioned putting a patch on the ground, and pop a cap. You should see the blade of grass or patch react, telling you the flame channel is clear. Another method is to point the barrel down and run a brush in and out of the bore to knock the fouling loose, but with the barrel pointed down it falls out of the bore rather than being pushed down to the flame channel.
I shoot at a range, so popping a cap has to be with the barrel pointed down range. Still works most of the time, but you can't point the barrel at something to verify.
Holding the rifle barrel down and running a brush through the bore is not an easy juggling act either at the range, plus I have a sidelock and can't easily remove the breech if I get a brush stuck in the bore.
The third option, and my favorite, is to turn the jag down a bit. I simply chucked my jag into a cordless drill and spun the jag on a flat file. I found a couple of nuts that I could put on the threads so I didn't damage them. I went a little at a time and tried to give it a very slight taper, smaller diameter in the front to an increasing diameter in the back. I did this while out shooting (not at a range) so I could check the fit on a fouled bore as I went. You are looking for a fit that allows the patch and jag to go down the bore smoothly, but then the patch will still bunch up and pull the fouling back OUT of the bore. The best jag for this is one of the slightly longer ones they sell that has a tapered shaft that gives the patch a place to "drop" into on the way down the bore. It took me a whopping 10-12 minutes to get the jag to where it was perfect.
Now I can run a slightly damp patch (nothing can be squeezed out of them) down the bore (I use Hoppes BP cleaner or Birchwood-Casey #77, I bought them to try as a wet patch lube for roundball shooting and just want to use them up) pause for about 8-10 seconds to let the fouling soften a bit, and then pull it back out of the bore. If it is a rainy or high moisture day where the fouling stays really soft I will run a dry patch, but if it is fairly dry/warm day I don't even bother with a dry patch most times.
It is a very rare occurrence for me to get a misfire now. I bought a second jag to leave at the factory size so it is very tight fitting and I can use it for when I do my thorough cleaning after a shooting session.