As far as cleaning goes I use dry patches every one to two shots. At the end of the session I use wet patches with solvent. I don't use dry patches to get the solvent I let it stay in the barrel for the ride home. Once home I stick the barrel in hot water off the stove. I swab the barrel letting the water blast out of the fire Channel. After the crud stops coming out I pull the barrel out and start running dry patches and dry the exterior of the barrel. If it's going to be stored for longer than a day I will run a patch lightly coated in bore butter followed by a dry patch to remove the excess. I start range sessions with a dry patch to clear out the bore butter. This method has always been effective until I had to replace the nipples.
Add this into your cleaning regimen, and it will solve your problem: Go and buy or order some of those cva/traditions pipe cleaners. Remove your nipple, Cut off an inch and a half piece, Wet one with whatever solvent you use, hot water is excellent if that's what you use! Bend the piece of pipe cleaner into a curl and feed it down DEEP into the spark channel, and scrub back and forth REALLY GOOD. Cut another one and bend it again if the first one gets all bent up and stops feeding.
This is really the only part missing from your good cleaning regimen. Its just that running water through the spark channel isnt enough on its own. The spark channel needs a good scrub followe by your rinse of Hot water or solvent. Blasting it through with a good jet is an excellent way to rinse it, but does nothing to scrub away the crud. You need a brushlike object for that.
After cleaning and oiling, before putting away your rifle, use one more pipe cleaner, this time dry, and swab your spark channel to make sure there's no remaining oil, water, or solvent in there. The gun will definitely go off the next time its loaded.
Now, you may need to spend an hour or two cutting and bending pipe cleaners to get caught up on all this gunk buildup in there, but from there on out it gets easier. Couple of passes each time you clean the gun in the future. And i dont expect any problems. I have never owned a rifle i couldn't get to go off. And I've had some real beaters in my day.
Just remember, crud catches sparks.
Hope this helps,