[nathan_hui]

Could you write some very basic diagnostic test code (run this motor continuously, run that motor continuously, etc. but only one thing at a time) and see if you still have the problem?

You can't be sure that the NXT isn't sending the command to the motors until you explicitly tell it do only that and nothing else (by writing a program to do only that). If that doesn't work, replace the HT Motor/Servo Controllers. Also test the motors by connecting directly to a +12V power supply. If you happen to have a function generator, you can also test the servos to make sure they work.

When you debug a problem, look at the way the robot receives it's control commands and simply follow the same path, testing each part until you find the one that doesn't work. That's essentially what we're doing here: testing in sequence the network, software, motor controller, and motors.