Change your bobbin. Mine are plastic and I have found that they get old, I had one that was full and giving me nothing but problems (they also have to click - you know the feel). I found that I had a mess on the underside of my sewing and the thread even came up to the top???. I wound a new bobbin (brand new), and had no problems. Pulled all that nice thread off my problem bobbin, but still didn't see anything. Closed my eyes and ran my finger around top, bottom and the middle and finally found with my fingernail a small crack. Dumped the bobbin and that was the end of my problem. So maybe it's as simple as that. Good luck