I'm assuming you're quilting with the feed dogs up. You might check the height of the presser foot. If your quilt sandwich is thick, the presser foot could be preventing the feed dogs from moving the fabric correctly. Also be sure your top tension is not too tight.
If the feed dogs are down, the stitch length setting doesn't mean anything. The length of the stitch is determined by how you move the fabric in coordination with how fast the needle is going up and down.