Sometimes the tweezers just need a but of tweaking to be aligned properly and set up before use. This technique can also be used when someone drops their tweezers and bends them.
Take smooth needle nose pliers, and squeeze the tips together to try and get more of the foot to clamp tight. Each side can be slightly straightened so the angle is a bit less and it closes more entirely. This can and should likely be done with any tweezer to set up its sweet spot for optimal lashing based on artists preference. If your tweezers have flattened out and lost their spring, you can close the tweezers with a pencil placed half way between the hinge and points to increase the bend. After this trick they should grip again.