Have you tried the Nice Menus module?
Nice Menus enables drop-down/right/left expandable menus. It uses only CSS for most browsers, with minimal Javascript for IE6. (Version 2 uses the Superfish jQuery plugin for all browsers, with an option to disable JS, and falls back to CSS-only for browsers that can handle it.)
Though it just might be a CSS error. if your submenu is position: absolute, check if its container is position:relative.
Also, if you added a z-index style in your css, it needs either position:relative or position: absolute to work properly.
