Hi Nic,
First I would check for internal leaks in each hydraulic motor (port L). Usually we should have leaks of approx. 0.5 liters/min.
If everything is OK, I would check the output from the PLC for the forward solenoid and the reverse solenoid, plug X3.1 (pin 1+2 white color, positiv for forward solenoid) and (pin 3+4 brown color, positiv for revers solenoid). The PLC board may be faulty.
Anyway, the guys at Combilift will help you, they're pretty pro.