Fourbore....I think you powder assessment is fairly accurate. Pellets are the worse culprit I think but the 777 powder will also crud up a gun as will some of the other sub pellets of similar chemical composition.
I've had some minor issues with the 209 powder, all of which came with the use of the "muzzleloader specific" primers. I tried shotshell reloading primers in Federal, CCI and Winchester's blue box and all of these erased the powder issues however the Winchester primers are yet and always have been the cleanest shooting primers in my guns. Since having gone to the Winchester primers about 5 years ago I have had exactly one hang fire. It was on a 10 below afternoon in a .45 Kodiak that I range shot about 12 times a couple days prior to the season and decided to NOT clean the gun. The plug was heavily caked up and that caused the mis-fire, not the primer or the powder. Of the three reloading primers I've mentioned I think perhaps the Federals are the hottest but the Winchester primers do an excellent job of igniting the 209 powder in the worst of cold weather for me and are way cleaner than the Federals. I think everyone develops a preference for certain things like primers and I also think there is no set in stone right or wrong in it. Primers are cheap so I suggest you pick up four or five different brands and shoot the to see which trips your trigger.