Sounds like you're holding back the fabric as your machine tries to sew. Be sure your fabric is pressed properly. Pin if you want to, and mark a 1/4" seam with tape on the machine if you need to. You might want to turn down the speed on your machine if you can. But to get it to stretch, there has to be more tension on the fabric than if it were just freely going through the machine, so you may have to analyze what you are doing.