CLK door lock trouble.

When I lock the doors with the metal slide out key by placing it in the drivers door (can only be done on the drivers door) only the drivers door locks and unlocks. It moves manualy and not power assisted.
Central locking by using the dash switch works fine.
Using the remote function on the key works fine too.
This is a recent development. When the car was new, both doors would lock and unlock manually with only the slighest turning of the key.

Any ideas where to start looking ?