What powder were you using, did you use plug grease, and did you hand tighten or wrench tighten?
Best scenerio is if you used BH 209, shouldn't sieze up and should come off fairly easily. With other powders, you can have trouble geting it off, especially if you tightened snugly with a wrench. When using other powders, I use the wrench, but hand tighten. If you make it snug with a wrench, it is a bear to get off after a shooting session.
Not sure on what to soak it in if it is seized up. Check with guys on mondernmuzzleloader website, some guys who know more than me there.
txhunter58
venor, ergo sum (I hunt, therefore I am)