Why does the presser foot drag the zipper while sewing?

Q: Why does the presser foot drag the zipper while sewing? A presser foot that is too heavy, worn or the wrong shape will drag the tape and distort the seam. The drag shows as an uneven stitch length where the zipper passes under the foot, which is easier to see than the drag itself. Run a scrap seam and look for changing stitch length where the zipper passes under the foot. Check foot pressure and whether the foot is level on the raised zipper. Inspect the foot sole for wear, burrs or a groove that catches the tape edge. Try a foot with a cut-out or a narrower sole designed for zippers. Check that the operator is not lifting or forcing the foot over the slider. Uneven stitch length that appears only over the zipper is a foot or pressure issue. A foot that fits the zipper profile, with pressure light enough to hold without crushing, is the usual fix. Change one setting at a time so the result can be attributed and repeated on other machines.