It is dirty, the oil has dried up on it and it needs a cleaning. As the movements in cuckoo clocks are open to the air around them through all the openings in the case they require more frequent oilings and cleanings. The dust from the air dries up the oil and clogs up the movement. I'll bed if you apply slight pressure to the weights that the pendulum has power to swing. This is a sign of a dirty movement.
Stevens Clocks, April 2010
