What size of stitch were you using? When sewing elastic I use the 3 step zigzag stitch and setting it at a long length. Instead of looking like zigzag it will look curved. Also, do not hold the elastic too tight while sewing. I usually use a stretch needle or universal for sewing elastic. If it is a thick elastic I use a size 14. I usually have the tension knob at 3. Hope this helps.