[philso]

Check your electrical connections from the PDP through the motor controllers to the motors.

If you are using PWM control of the motor controllers, disconnect all PWM cables except for one motor controller on each side. Verify that both sides run. Disconnect those two PWM's and connect the PWM cables for another controller on each side and repeat. Verify that both sides run.

Reconnect all PWM cables. Command the robot to drive straight for some fixed distance, say 10 ft. measure how much the robot has deviated from a straight line. Repeat this measurement another 4 times. Swap the connections between motors and the controllers so that the controllers for the left are now driving the motors on the right. Don't change the software. Command the robot to drive straight for the same distance and measure the deviation from the straight line. Repeat this measurement another 4 times. You should then be able to tell if the problem is in the motors or mechanical system or if it is in your software or motor controllers. If the problem appears to be in the motors or mechanical system, swap the motors from one side to the other and repeat the measurements.