I have the 6600 and no problems with my bobbins. I assume you've taken the plate off, taken the bobbin case right out and given a good clean all around the works with a brush? Otherwise, perhaps you are not winding the bobbin correctly - you have to thread a length through the little hole in the bobbin and hold it between you fingers then wind a few turns to secure it. Stop the machine, trim the thread end off then push the button to finish winding. I'm sorry if you ARE doing this then ignore me. :-D