I taught the Double Wedding Ring for three years before I found the solution to this frustrating problem. The bias at the center of the curve has very little "give" but really stretches as it moves to the narrow ends. So, when you sew from the center to the end it works well but if you sew from the end to the center it pushes the bias to the center, creating a "bulge". Secret - pin the two pieces at the center and the ends. Sew (or pin) starting at the center and stitching to the ends. It'll be perfect!