I used to have that happen. I has to do with how the fabric is folded...not straight. I fold it once, pin across the top, press, and then fold it again for approx. 10" widths that fit under my June Taylor Shape Cut ruler. Don't try to match the ends when folding, hold it so that there are no waves in it and pin and press. They don't cut them straight at the fabric stores. Then square up the end and cut from that. That should help. I saw this method once on Simply Quilts.