if this truck is equipped with fwd/rev contactors make sure they are making contact.
engaging fwd might be seating the normally closed side of the fwd contactor tips.
as chevotaman said above.
so what it may be is just a switch problem where you need to actually go through nuetral and you don't do that unless you twist back from forward switch engagement.
Also Keep your safety reverse switch (red belly bump button on control pod) circuit in mind. Are you getting fault code when the truck fails to travel when commanded to do so?
Which controller do you have? Curtis, SEVCON, etc? Do you have a handset for your controller? Just off the top of my head based on info provided you may have contactor issues??? Sticking/welded/burnt tips? If memory serves this model had directional contactors, 1-A and anti-tiedown I would start there.