Well....I guess you could say there is a definite problem with the description of the problem.
To my way of thinking, the bolster is the part of the lock that rests up against the side of the barrel.
After re-reading your post several times, I'm wondering if your gun has a seperate percussion drum with the nipple screwed into it, and the threaded part of this drum that screws into the barrel is interfering with the front of the breech plug???
Actually, your question is better served in the Gun Builders Forum, and I will move this there in a day of so. I'm mentioning this so you won't think it got lost.
If my guess (up above) is correct, the nipples flame channel needs to have a clear passage to the bore of the barrel. If it is half blocked off, the material that is creating the closure needs to be removed.
Before you start grinding material off though, you need to make another stab at describing exactly where and what the problem is.
zonie