No COM for Arduino Robot's Motor Board

The control board on top is functioning perfectly fine. I can upload sketches and stuff. But the motors are behaving oddly. So I wanted to reupload the motor core sketch but to my disappointment, the robot's motor board are not being detected. I've tried on differect PCs to no avail.

How do I start troubleshoot this problem? Guide needed. Thank you.

Wanted to add that I've tried on different OS(ubuntu 12.04, win8, win8.1 and win7) too.