What sewing machine are you using?? It might be has simple has adjusting the height of the vibrating presser foot. I own my Singer 111W155 for quite a while and was getting some bunching and tighter stitch length when sewing 1/4" sew foam around radiuses. I finally figured out that my vibrating presser foot was only lifting about a 1/16". I dug into the manual and figured out how to adjust it and got it raising about 3/8". Now it sews much better on thicker sew foam and keeps an even stitch length with no bunching.