[Alan Anderson]

indubitably is correct. One of your motors is running backwards. You can fix that by either swapping the wires or by changing its "invert" constant on the Open Drive in Begin.

Put the robot up on blocks. Push the joystick forward. Observe which motor is doing the wrong thing. Fix it to do the right thing.

(If you then find that the robot is turning the wrong direction when you push the joystick sideways, that would be because you have the left and right motors swapped. Fix that in the Begin vi as well, by changing the left and right motor PWM definitions.)