I have a 403A. If it is the top thread, then it may either be a tension problem, or a needle problem. I would first look at the needle to make sure that is in new condition and put in correctly. I believe that the flat side of the needle faces the back. If it's a tension problem, I would check out the tension knob to make sure that there is nothing stuck between the discs and that the knob with the tension numbers on it is put on there correctly.
~Cindy