[DaveL]

If you are using dead axles (round bearings supported by bolts) make sure the bolts are a little loose. Like others have said, take the system apart and test each component separately and then as it is reassembled.

Is your robot's CoG centered side to side? If one side of your bot is heaver it may increase friction and cause you to turn in that direction.

As others have said, if your frame is not square, your robot won't track straight. If the bearings on one of your motors or wheels has higher friction, you bot won't go straight.

If its not mechanical, you could try using a 'Y' cable so each motor controller gets the same PWM signal.

You can program a button to pass both sides of the robot the same value or use a 'Y' cable to distribute the same PWM signal to the right and left sides of the bot. If the bot goes straight with this set up its likely your joystick controller is the issue.

The key to finding the problem, is to be able to rule out each component in a systematic fashion. Let us know what you try and the results.

Dave