Hi Sue,
I would make sure that you are using the correct type of needle for your fabric. Some knit fabrics require a different needle than sewing with cotton. Also if you're (cotton) needle was ready to be changed that could be a problem as well.
The other thing to try is to use a "leader" a small bit of fabric that you start sewing on, then push the new fabric right up next to the edge of the leader fabric and you should be ok.
Also are you careful about pulling up your bobbin thread and holding onto both the top and bobbin threads as you start to sew? This is also a good practice to develop.
Please keep us updated as to what you find worked to solve your problem as it may help another quilter in the future.
Good luck with your quilting projects.
Pam M
Also