Additional battery packs often come as either 4.5V (3 batteries) or 6V (4 batteries). The micro:bit will supply 0V or 3V on the PWM pin0, and this has to be above the digital input pin threshold of the servo. If the voltage supplied to the servo is too high, half of that voltage becomes the pin threshold and the 3V from the micro:bit might not be enough to direct the servo.