Module assignment issues

I set up a test site running locally - Mamp/Safari/Firefox - and had no problems setting up hidden menus to assign modules to articles without a menu.
However, on trying to recreate the site online (with a fresh installation) the module assignment no longer works when I try to place a module on the hidden menu item. However, if I select 'All' then the module does appear in the desired position, BUT if I select all the menus in with the 'Select Menu Item(s) from the List' button, it disappears again. I've even tried another fresh install on a different domain and the same thing happens.
Does anyone have any ideas what is going on here? Is something missing from the installation (Quickstart)? I can't see any differences between the local install and the remote ones - but whatever, there's obviously something amiss....

Hi

The only reason can be that a different page is visible uder the menu item. Please check if there are no two links to the same place (in such case the last instance will be udes only).