Navbar sub-pages only displaying on parent page

hi

I have a navigation bar with some top-level pages and 1 layer of sub-pages. However the sub-pages are only visible (on-hovering over the parent page link) while on the parent page. i.e. on the homepage I can’t see any sub-pages in the navigation.

I’ve been fiddling with my code below to no avail…

<ul id="nav">
                    <?php if (is_page()) { $highlight = "page_item"; } else {$highlight = "page_item current_page_item"; } ?>
                    <li class="<?php echo $highlight; ?>"><a href="<?php bloginfo('url'); ?>">Home</a></li>
                    <?php
wp_list_pages('sort_column=menu_order&depth=3&title_li=&exclude='.get_option('nav_exclude'));
		?>
                </ul>

Any help would be muchly appreciated.

cheers