yes check the contactors, if they are worn they may appear to be making contact but might not be making enough to make a good connection.
I'd also check the full range of the voltage readings comming from the potentiometer in both fwd and rev and make sure your getting full range of motion in both directions.

Also make sure there is nothing keeping the truck from moving in reverse such as brake dragging. Sometimes brakes can drag and you will get movement in one direction but not the other.