Usually you set your stitch length to zero. When you FMQ, you control your stitch length by moving the fabric. You don't use the feed dogs, so the machine can't control with the stitch length. If you have your feed dogs up, try it with them lowered. If they don't lower, tape an index card over them, with a small hole for the needle to go thru. If you have them down, try putting them up. While many people FMQ with feed dogs down, others have success with them up. Remember fast feet (run the machine at high speed) and slow hands (move the fabric slowly and evenly). It takes A LOT of practice! Also, this is one technique that you can really benefit from hands on instruction. See if a quilting shop in your area offers a FMQ class.