Q: What causes a zipper slider to lose grip over time?
The slider holds the chain closed with two flanges and a spring-loaded pull. Over time the flanges wear, the spring loses tension and the gap between the slider cheeks widens. Once that gap exceeds the tooth profile, the chain can slip out under load and the zipper opens behind the slider. Grit, sand and dried detergent accelerate the wear by lapping the metal or resin surfaces.
Pulling at an angle is the other major cause. A wearer who tugs the puller sideways levers the slider open, and repeated levering permanently spreads the body. A slider that is one size too large for the chain, or a chain that has stretched, produces the same symptom much earlier.
Diagnose by measuring the slider gap with a feeler gauge or by comparing it with a new slider of the same size. A slightly spread metal slider can sometimes be re-set with a proper tool, but a worn spring or a cracked resin body cannot be repaired. Replace the slider and check the chain for flattened or missing teeth; if teeth are worn, a new slider will only postpone the failure.