My van has a C200 toilet, when I had a problem like this it was because the small screw holding the mechanism in place inside the door hatch had fallen out, unfortunately you cannot get to it without removing the cassette but once it was removed I could see where the screw should be, it was at the top where you could see the hole of the toilet bowl, at the side was a plastic lever with a hole for the screw, I put it back in and still perfect 5 years later, if this isn't your problem there are videos on YouTube giving instructions of how to overhaul the cassette itself