I'm assuming that you have already removed the needle plate and checked for anything that might be jammed in the thread cutter. If it's clear, have your dealer check it out. There may be an adjustment required. The thread cutters on the 6600 and 11000 were tied to the rotation of the machine, but this one (and the 7700) has an independent motor. That should make it more reliable. Is this a new problem, or has it been happening all along? It is only with So Fine thread, or others as well? If it's isolated to the fine threads, tension may be a factor. The top thread needs to be held while cutting, but really fine thread might sneak through the tension enough to miss cutting. That's just my guess - the best course is to take it to someone who can actually lay hands on the machine!