I finished the Unimog from LEGO 2 or 3 years ago, and I noticed this issue, but now I'm getting pretty angry because I still can't find the problem. It seems to be that when I use the instructions for the function that should turn the crane, it doesn't turn. All other functions work but unfortunately the turning, one of the coolest functions, does nothing. Any recommendations?
This can have many causes, starting at the gearbox, followed by the transmission of torque to the turntable, the build of the turntable itself and as Joost wrote, the pneumatic hoses or other components can also interfere with the correct operation.
I would recommend following the path of the torque from the motor to the gear that turns the turntable and keeping an eye open for any locking pieces or possible sources of extreme friction.
If you can't find the cause, you could take some highly detailed photos of the aforementioned areas and I'll gladly check them with my Unimog to see if I can spot a difference.