Because the fabric was stretched as it fed and relaxed afterwards, leaving the thread slack — and tightening the dial makes it worse.
What happens. A knit under presser foot pressure is pulled slightly longer than its resting length. The stitches are laid into stretched fabric. Off the machine the fabric recovers, the seam shortens, and thread that was correct is now loose and wavy.
The fixes, in order.
Reduce the presser foot pressure if your machine allows it. This is the single most effective change and the one most people do not know they have.
Use a stitch with give — a narrow zigzag, a lightning stitch or a proper stretch stitch. A straight stitch in a knit seam either looks loose or snaps when stretched, and often both.
Fit a jersey or ballpoint needle. A universal point cuts the loops rather than passing between them, which weakens the seam and changes how the thread sits.
Support the fabric. Let it feed rather than pulling it through, and never let the weight of the garment hang off the table dragging the seam.
Use a walking foot on slippery or very stretchy knits, which drives both layers evenly.
What not to do. Tightening the top tension on a knit gathers the seam and produces the wavy hem everybody recognises. If the seam is wavy, the answer is almost always less tension and less pressure, not more.
Questions people actually ask
My hem waves however I sew it.
Stabilise it with a narrow strip of fusible tape before hemming, and reduce the foot pressure. A twin needle over stabilised fabric is the usual finish.
Ballpoint or stretch needle?
Ballpoint for ordinary jersey and interlock; a stretch needle for fabrics with elastane, where skipped stitches are the problem.
Last reviewed 2026-08-11