Why is my overlocker skipping stitches?

Change the needle before investigating anything else. Overlockers run fast and blunt needles quickly, and a blunt or slightly bent needle is the commonest cause of skipped stitches by a wide margin.

Then work down this list.

The needle type and system. Overlockers take a specific needle system, and fitting a domestic sewing machine needle where the manual specifies an industrial type puts the eye at the wrong height for the looper. Check the manual. On knits, use a ballpoint or jersey needle — a sharp point pierces the loops and skips.

The needle position. Pushed fully up until it stops, and in the correct orientation for your machine, which is not always flat-to-the-back as on a sewing machine.

Threading. Re-thread completely, in the order the manual gives, with the presser foot raised. A thread that has slipped out of a guide or a tension disc skips exactly like a mechanical fault.

Thread quality. Old or cheap thread with slubs and inconsistent thickness snags at the looper eye. Try a known good cone.

Presser foot pressure. Too little pressure lets the fabric lift with the needle, and a lifting fabric skips. Very lofty fabric does the same.

Tension far too tight on the needle thread will prevent a clean loop forming for the looper to catch.

Only after all of that does looper timing become the candidate. It is genuinely rare, it is a workshop adjustment, and attempting it yourself on a threaded four-thread machine usually makes things worse.