Flushed with success - not ! Basic query

I'm sure the answer to this is obvious, but as newbies, we can't solve it !  We have a Thetford Cassette Toilet, which worked well for the first one-night trip we made. (all prepared for us by the dealer.) However, we now find, when trying to flush, it makes a noise but no water comes out. On investigating the filling tube outside, there is still water visible in it, and no more will go in. (We've also emptied the waste out, thinking that might help, but obviously not !) What are we doing wrong ?