When you say nothing happens, do you mean visually, or according to the status of the gripper?
On our real krc4 (without gripper tech) to grab: i have to first set release to false the grab to true. to release: I set grab to false then release to true. mihg tbe worth a try.
Im running Sim 3 and OL 8 (not properly connected yet) and had similar where blocks weren't being picked up, think it was down to physic, could be the same for you