First make sure that your can manually increase claw closure
You may have reached the mechanical end stop of your left servo
Just unscrew, lift the driving arm and rotate it
i thought it might be because of the my limits being the oposite to yours (close is higher than open, yours is the opposite)
That's a possibility
Remove cube,
adjust DOWN/BACK_CLOSE/OPEN parameters one at a time and see the effect for yourself
For a good grip, without cube inserted, claw should close about 2mm more than the actual cube edge size and open 1 to 2mm more than cube diagonal