I have a Thetford C2 Cassette which when I rotate the handle ( shaped like a jar lid ) the flap that revolves to open to allow access to the toilet does not open fully. It turns round so that it is about 2/3open. I have not noticed this before so am not sure if this is normal, I would suspect not as stuff could get lodged half way through the hole. Anyone have any knowledge of how the flap works.