Why is my sub-menu not loading in main menu?

Question

<div class="row" id="mainNav">
 <?php print theme('links__system_main_menu', array(
    'links' => $main_menu,
    'attributes' => array(
      'class' => 'menu',
    ),
  )); ?>
</div>

I've used the above code to load main menu in page template. This code fails to display submenus. Please suggest.

Answer 1

For Drupal 8:

You must configure the block settings of Main navigation or any other menu. The default setting for Maximum number of menu levels to display is set to 1. Change this setting to Unlimited to see children displayed.

Answer 2

1. Go to /admin/structure/menu/manage/main-menu
2. Edit your parent link
3. Enable "Show as expanded" checkbox
4. (Repeat the same for all the parent links)
5. Go to the front page and see the main menu expanded.

Update:

You can use next code to render menu:

 $main_menu = variable_get('menu_main_links_source', 'main-menu');
 $tree = menu_tree($main_menu);
 print render($tree);