OWI-535 robot arm

I am using a OWI-535 robot arm for science Olympiad and after I put it together only half the motors work. The motors that do work only go in one direction. I have to unplug the wires and flip them around to make it go the other direction.
Do you know of any easy fixes or problems that could be causing this?

The manufacturer of the 535 requests that all issues with their products be sent to them directly: 310.515.6800 or [email protected]

Thank you. I was having trouble finding the manufacturer’s website.