The ItemListbox does not redraw itself on some cases, like when adding a new item. I haven't been able to find exactly what causes it: adding an item may or may not cause it to redraw!
Additional information: Originally reported on the forum, here: http://www.cegui.org.uk/phpBB2/viewtopic.php?f=3&t=6220
Using the Ogre renderer, resizing the window (by dragging its border) is the only way to make it redraw and show the items.
OS: Linux, OS build: Arch, Platform: x86_64