"Show as expanded" selected but children menu items not displaying
Bartik theme is not designed by default to show parent items in Primary navigation section as expanded, even if you checked "Show as expanded" for each parent item in the main menu.

In order for you to make Bartik theme to render parent items and their children items; add the following code to template.php themes/bartik/template.php

function bartik_links__system_main_menu($variables) {
  $menu_name = variable_get('menu_main_links_source', 'main-menu');
  $menu_tree = menu_tree($menu_name);
  return drupal_render($menu_tree);
}


Note: make sure to clear Drupal cache once you apply the new changes.