What kind of presser foot are you using? When doing a satin stitch it is better to use an applique presser foot. The groove on the bottom of the foot allows the thicker zigzag stitch to pass under the foot without it getting bunched up---keeping your feed dogs feeding your fabric through smoothly.
Also, make sure that your stitch length is not too short---a short thick satin stitch will keep your fabric from feeding evenly.