When you use use a lot your "third hand" you turn the grippers to different positions quite frequently. Thus, it is possible that the fixing of the grippers wears off. They wont support in position any part / board having a certain weight.
To solve it, I just solder them! Grippers are made on a soldable alloy. The arm is not soldable, but if you apply the soldering iron for a while, the soldering material will flow between the two parts. Once cooled, they are fixed forever!