Sometimes when you are satin stitching on an applique the pressure on the presser foot needs to be lightened up a little, and it always helps if you have a satin stitch foot. (They have a cut out depression on the bottom of the foot that allows the stitches to slide under easily.) Also if you pack your satin stitches too tightly it will cause buckling, so the thing to do is a test sample first and use JUST ENOUGH stitches to get the coverage.
