holmes4:
Check which pins your using for servo timing and motor control. For example, the standard servo.h lib is not compatible with the standard motor shield.Mark
The bot uses two servos to move it one for the left and one for the right wheel. Those two servos have some modification that makes them a continuous rotation servo motor, they also have a potentiometer for calibrating the speed so they both move the bot strait. But it can be a pain to keep the two servos even and calibrated.