Thetford C4 cassete toilet, water on cassette?

When the toilet has been used and I go to remove the cassette there is "let's say water" in the handle wells. I renewed the main seal and tried it again after a few flushes I checked the cassette and there was water in the wells again. When the cassette is pushed into position everything seems OK but I have noticed the cassette will move back and too about 20mm is this right. and can anybody give me a clue, Bob!