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.

Hi Nic
Not sure of the specific issue and or age of your truck, but Combilift have a dedicated Technical Team covering North America. If you still need support, you can complete a quick technical support form at the bottom of the main combilift.com web site or direct link https://warranty.combilift.net/technical-support-form/. The form only asks for the minimal info so the team can quickly help you. There is no login required and no charge for this support.

The North America Team will be back to you quickly. Hope you get sorted quickly.

hi there are many things that can stop the unit driving but if you have directional change working just a question make sure brake light is operating on the dash when you put handbrake on i have had trouble with the switch under the inching pedal which also activates the brake just a thought richard from combilift will help you when he sees the post
rule of thumb dont over think the problem some times its easier than you think