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Did you ever take the legs apart and look at the locking mechanism?  It's as simple as a metal cam that is spring loaded. When you pull the trigger the cam is released and the legs can move. When the trigger is released the cam turns and locks. If you lost friction, I see two options. There is a plastic bushing around the leg where the cam is positioned, to center the leg. It is broken and leaving too much clearance for the cam to successfully lock the leg, or the cam is pushed down and not able to turn against the other leg.