I also chain stitch a lot on my 440 and don't have a problem. I would second Tartan's advice and check the thread path and the type of thread, making sure to use the right spool pin (horizontal for cross wound, vertical for straight wound). You didn't say if you tried different thread? What Bernina send just sounds silly. The bobbin case clicks into place. there isn't back-to-front play in it's position. I know you sound frustrated. I've had my 440 for 6 years and it has never let me down. The embroidery unit, that's a different story!