I've had that happen before and I alway find it to be in my bobbin. I remeove the bobbin case, it sometimes, when working with various fabrics will get off kelter, sort to speak. I remove the bobbin case. Make sure to clean all lint and threads that sometimes collect under the case as well as inside the casing. Always clean with a dry soft brush, I never oil the bobbin case or surronding area of bobbin case, check your tension, and rethread. That has helped me on more then one occassion. Your sewing machine manual is the Bible for your machine and keep it next to your machine, it helps to keep you on the right track and to really learn your machine. I even make notes in my book as to stitch combos, or/and problems that I write the solution to. It's like learning the ways and and cries of your new baby. Sorry I sound fanatical, but I am when it comes to sewing.