I have had a similar problem, don't have your machine; but all of them have their quirks. Each time I encountered the problem it was the top thread. How it happens I haven't a clue, but just taking it out and doing a cleaning and then re-threading after checking the bobbin and cleaning that also has always solved the problem. I just finished a project in which I had the thread breaking and couldn't figure out what the heck was going on. And as Peckish said, as a doorknob I had put some patterns on the right side of my machine and they were laying up against the thread spool. Duh, sometimes I wonder if I should be allowed to play with sharp objects.