I am no expert. If it sounds like it's trying then we assume the solenoid valve is okay? That leaves the clamp closing speed knob on the front panel and the hydraulic interruptor. You have checked the knob isn't set to zero?

If b20 lights the interruptor roller must be dropping into the notch on the rod. It has a perspex window in the front, you should be able to see the switch operate when you open and close the guard.

If it was me I'd probably divert the interruptors return to reservoir connection in to a bottle and see if it filled up. If it did, I'd disassemble, look for a blown seal, then consider bypassing the blessed thing if faulty, it looks kind of expensive to replace. Rather depends on your H&S constraints.

Would a movie of it opening and closing help? There might be clues in the sound track.