Remember when there is a problem with the underside of your stitches it is usually the upper thread and not the bobbin PaperPrincess is most likely right in what she said. Check all threading and make sure nothing has changed.
Originally Posted by
PaperPrincess
Are you sure it's a bobbin problem and not that the thread has come out of the uptake lever?? That's what it usually is with my machine. There's a glop of thread on the underside of the fabric, and the thread has jumped out of the lever.