The only way that the reverse arm wouldn’t move when trying reverse would be the selector pin not engaging the hole in the reverse lever. I don’t think there’s an alignment hole for a rod or bit in the reverse lever on a stock shifter so I would say if everything is right in the shifter the reverse lever on the shifter is out of position.
Do this unhook the reverse rod and move the shifter over toward reverse and move the reverse lever back and fourth until the selector pin engages. You will need to hold a bit of pressure toward the detent spring to feel it going in the lever hole. Next do move the reverse lever and adjust the reverse rod to the lever.
