Be sure your presser foot is up when you thread the machine. If not, the thread doesn't properly seat between the discs. I had the same intermittent problem for years and couldn't figure out why. Finally did a Google search and saw this suggestion and have never had the problem since. The reason it was an intermittent problem was because sometimes the presser foot was up while threading and sometimes it was down, but I never realized the connection. It just seemed that sometime the machine worked great and the next time it didn't. It was VERY FRUSTRATING!!