I may have jumped in a little late and couldn't read all the possibilities of the problem. What I did skim, there's lots of good advise.
I, too, sometimes use a bronze bore brush. If you do, point the muzzle down. The brush won't gather the crud and pull it out. It deposits it in the breech, especially when you reload. The vent lets the air out as the load is pushed in. Any particles will go with the air and plug the vent. Try it (if you want to pull a load again) and check the vent. Many times there's a little hard piece of crud in the vent, which may happen in a patten breech, also.
Just my $.02
Good luck and don't give up :thumbsup: . Once you find what it likes, it'll run so smooth you may forget most of these problems.
I, too, sometimes use a bronze bore brush. If you do, point the muzzle down. The brush won't gather the crud and pull it out. It deposits it in the breech, especially when you reload. The vent lets the air out as the load is pushed in. Any particles will go with the air and plug the vent. Try it (if you want to pull a load again) and check the vent. Many times there's a little hard piece of crud in the vent, which may happen in a patten breech, also.
Just my $.02
Good luck and don't give up :thumbsup: . Once you find what it likes, it'll run so smooth you may forget most of these problems.