If you open the clock on the back you will see a thin wire that starts at the bellows and goes up to the cuckoo. This wire should move the cuckoo up and down a little while it calls. It is not connected with the cuckoo but usually ends under the cuckoo’s tail (see picture).
During shipping it may happen that this wire is turned above the cuckoo. If it is above, it may block the cuckoo and the door.
To fix this you can simply turn the wire around the cuckoo – carefully – so it is under the cuckoo’s tail again. This should be quite easy if you open the cuckoo’s door (the cuckoo will move forward when the door is opened).