I just ran across an old article by Diane Gaudinski, and she mentioned that the foot should "bounce" with each stitch and that you should be able to move the quilt sandwich when the foot is down. She said that the foot on some machines may not allow this, and to try the embroidery foot, the quilting foot and the darning foot to see if one was better than the others. She also mentioned adjusting the pressure on the foot if you have a machine with this adjustment. With thinner batting, the pressure may need to be increased; with thicker batting, it may need to be decreased so that the quilt sandwich can be moved when the foot is down.