I purchased a set of these clamps and constantly had the problem of them locking up when you squeezed the handle to move the rear clamp to tighten.
I stopped using them, but couldn't bring myself to throw them away.
Today I decided to take one apart and find the problem.
HERE'S THE FIX:
The problem is that the squeeze handle goes back too far into the hollow handle slipping off the spring clamp inside the head of the clamp.
If it is locked up, remove the screws from the side of the clamp and re-position the squeeze handle.
Then cut a piece of 1/2" plywood 11/16" wide by 1" long.
Place the piece of plywood into the hollow handle behind the squeeze handle.
Drill a small hole throught the handle plate into the piece of wood.
Insert a small screw to hold the wood in place (counter sink the hole and usie a flat head screw for a clean look)
Now when squeezed the handle won't go back to far, slipping off the spring clamp.