Drop down with sub items truncated when on the right hand side of the window

Hi, I'm using the drupalexp_menu and I have a "more" menu item on the right hand end of the menu - this will have lots of extra menu options including some extra sub menus - in the screenshot below the articles menu is a sub menu but the items aren't listed - nothing is shown - anything that would expand out to the right is truncated at the side of the page. See screenshot. Any help appreciated. Thanks, Ray Smith