Changing Banner Adverts in block?

I'm setting up banner blocks at the moment.

I presumed I would set up a block then assign particular banner adverts to that block - and they would be displayed on rotation (ie every time the customer visits a new page or product, a new advert is displayed.

…is this incorrect?

I can either get 1 advert continually - or a number of advert stacked on top of one another!

I just want a single advert (in any one place) - but want the adverts to change as pages are changed…

Am I doing something wrong - or does CSCart not work like this?