The top navigation menu in Novus theme is using Split Menu theme which shows the first level of menu in the top and the second level in the sidebar. So, the theme does not support drop down menus, but you can add it to the theme if you want. First, open the header.php file, and on line 148 remove the "depth" parameter, like so:wp_nav_menu( array('menu' => 'Top Navigation', 'container' => 'ul', 'menu_class' => 'menu', 'fallback_cb' => 'rok_old_menu', 'link_before' => '<span>', 'link_after' => '</span>' ));
after that you need to add some CSS styling to the theme to make the drop down menu work.