starch the fabric well... bias edges tend to stretch easily- starched and the broadcloth on the bottom with the other cotton on top may help (or the other way around if that way is causing the problem) a nice new sharp needle, I would not adjust the tension but I would lengthen the stitch length a little bit--- many of joann's fabrics have a bit looser weave than fabrics you will find in a fabric/quilt shop...so, pre-washing, a good starching/pressing will help tighten the weave and add some stability.