I have not tried to do a decorative stitch with my walking foot engaged, but as Tartan said, that may be the problem. Other people have indicated that some machines and some walking feet refuse to anything but forward when working together. Some machines won't zigzag with the walking foot on. Mine will do that.
Remove the walking foot, do your fancy stitch then replace the walking foot and straight stitch. If both of these are successful, then it may be the combination.