Why does the zipper lock fail and slide open by itself?

Q: Why does the zipper lock fail and slide open by itself? A lock failure is either a worn locking pin, a spring that has lost tension, or a slider fitted to the wrong chain. It is a function defect rather than a cosmetic one, and it should be judged by a simple hand test rather than by appearance. Close the zipper and push the slider along the chain by hand; a working lock resists. Check whether the lock holds in one direction only or not at all. Inspect the locking pin and spring for wear, bending or debris. Confirm the slider type matches the chain, especially after a replacement. Test several samples from the same lot to see whether the failure is isolated. A lock that fails across a lot is a maker issue and should be raised with the lot number. A lock that fails after a replacement slider is an assembly issue. For garments where the zipper must not creep, the lock type should be confirmed at the design stage rather than corrected in production.