No, Not an SCR problem if the right side is working. If the contactor picks up for the left side and you have no travel I would be looking for a problem in the power circuit like traction fuse or brushes. With the truck blocked up off the floor you could push in a directional contactor for that side in question and then the 1A contactor, that motor should run or you have an open circuit on that side, like a power fuse or motor brushes.