Wood changes size and shape with temperature and humidity. Even small changes in shape can disrupt the workings of gears. Therefore -- and this actually is starting to happen as the summer arrives -- wooden clocks can stop working when the weather changes. I've had to restart my clock a few times over the past month on particularly warm and humid days.
[Edit: It seems like humidity higher than 75% stops the clock and it won't restart. Below around 50% seems to let the clock run reliably.]