Why has my stitch length stopped changing?

A stitch that will not change length is usually a feeding problem rather than a dial problem.

Check the feed dogs are up. Dropped feed dogs — for free-motion work or button sewing — mean the fabric only moves as far as you move it, so the length control does nothing at all. Also check that a darning plate or cover has not been left fitted.

Check nobody is pulling the fabric. Guiding is fine; pulling overrides the feed entirely and produces long, uneven stitches whatever the dial says.

Check the presser foot pressure. Too little pressure and the feed dogs cannot grip, so they slip rather than advancing the fabric a set distance. Very heavy or lofty fabric does this too.

Check for lint in the feed dog slots. Packed lint holds the dogs low so only the tips contact the fabric. This produces short, dragging stitches on every setting and it is very common.

Check the stitch selected. Several stitches — buttonholes, some stretch and decorative stitches — set their own length, and the dial then has little or no effect. That is designed behaviour.

Check for reverse-feed engagement. On some machines a stitch length dial pushed past zero engages reverse, and a dial sitting near that point behaves oddly.

Then a mechanical fault. A dial that turns freely with no resistance and no effect suggests a disconnected or worn feed regulator, which is a workshop job.

Test on a doubled scrap at 1 mm and at 4 mm and compare. If both look identical, the fabric is not being fed at all — go back to the feed dogs.