I'm going to say something that you already know and that is; You should not be having this problem. If you followed all the basics, then perhaps the next step, is to contact Lyman and see what they have to say. .....
Now then, I personally have not gotten into the Blackhorn-209 but some of the folks around here, that are, are drilling out the vents, on the nipples. I don't recal what size drill but can find out, if you wish. .....
HH, Yes I did fire a primer for to dry out the breech and barrel.
Pahoo, I've hunted with a CVA Kodiak Magnum for the last five years. I know the basic and then some. It was too late to contact Lyman on the east coast when I was trying to shoot. I did have a call into Western and was waiting for their call back when I made my first post.
I later talked to Doug at Western. He said the Lyman breech plug design was not compatible with their Blackhorn 209 powder and recommended I try 777. He also has a Lyman Mustang muzzel loader and has to use 777.
Tomorrow I will be taking to Lyman to see if they have a different breech plug that will work with Blackhorn 209 powder.