Lucy, my first suggestion is to check the stitch length. Maybe your stitches are too tight. Secondly, are you letting the feed dogs do their job of moving the fabric? Never push or pull your fabic strips. Thirdly, I would recommend you look into aquiring a 1/4" foot for your machine. This foot has a little flange on it to keep the fabric straight.
I hope you find this information helpful. Please let me know if you have anymore questions.