It is possible that the gear you have is not homed correctly in the copper lever which is under tension from the silver fork spring. When fitting the calendar gear, work with the stem away from you @12, position the gear fork at 1 o'clock and the circle hole @10, and then release the the copper lever gently, and then it should engage the underside of the calendar gear correctly. You will need to use very fine tipped tweezers to hold the copper lever back whilst placing the gear in its pivot, ensuring that part of the gear is under the shield which slides/locks onto the main plate pivot, the same as the copper lever under it.