I have had this problem also and then started using the other side of the ruler(until I get a new one) Also I found that if I folded the fabric more that twice, I would get that "bow". When you buy fabric, it does not come of the bolt in true grain, that could also cause the problem. To get the true grain, cut on the original fold, about 1/2 inch and rip. Then trim the raw edge. Bearisgray has some great advice. I am going to buy a new ruler.