First, as has been said here, do NOT file the breech plug face down two threads or any more for that matter, if the face of the plug has reached the beginning of the bore, i.e., bottomed out. Next, open up the notch a little bit with a file but, and I am pretty sure this is going to happen, unless you open it enough and pay constant attention to the fouling build up at the touch hole, you will have more problems after firing it very much. Next, check the size of the touch hole and whether or not there are burrs in it. If it appears too small then open it up a bit or if there are burrs, just clean them out. You might consider coning the outside of the touch hole as well. If all else fails, can you get your money back? By the way, who built your gun?