My boss at the LQS brought in a Singer 403 for us to use at the shop (we did have a Stylist Touch and Sew, but the bobbin winder was giving us fits). She said it was working fine at home but now it keeps breaking the top thread. We've tried everything and just can't seem to figure it out. There was a thread stuck under the bobbin race, thought that was the problem but it didn't change anything when she pulled it out with tweezers. Any ideas?