I found this at http://www.snapsource.com/faqs/didyouknow.php
you can tighten or loosen a socket tension to eliminate "gaposis" when a snap doesn't stay closed. This problem is most frequently found on ready-made children's clothing or adult robes. Simply use a needle-nose plier to tighten the tension teeth by pushing them in slightly and push the teeth gently outward to loosen the teeth. It's just that simple -- just remember a little movement goes a long way. Don't overdo it!!
Denise MT, January 2011