title: How to add new custom submenu under another plugins menu
body: <p>There is plugin called Shopp in my WP admin page , this plugin has got top level menu "Shopp" .</p>
  
  <p><strong>This is the top level menu:</strong></p>
  
  <pre><code>$menus['main'] = add_menu_page('Shopp', 'Shopp', SHOPP_USERLEVEL, 'shopp-orders', array(&amp;$this,'orders'));
  </code></pre>
  
  <p>And I've created some plugin which need to add as submenu under "Shopp" top level menu , so it is adding sub menu ( link .....wp-admin/admin.php?page=ach-faq.php ) but when I am clicking on submenu it shows "You do not have sufficient permissions to access this page."</p>
  
  <p><strong>Debug result:</strong></p>
  
  <pre><code>Pagenow = admin.php
  Parent = shopp-orders
  Hookname = shopp_page_ach-faq
  Menu = Array
  Submenu = Array
  Menu nopriv = Array
  Submenu nopriv =
  Plugin page = ach-faq.php
  Registered pages =
  </code></pre>
  
  <p><strong>My code:</strong></p>
  
  <pre><code>function ach_faq_menu(){
   add_submenu_page('shopp-orders', 'My FAQ Plugin', 'My FAQ Plugin', 8, __FILE__, 'section_1');
  }
  function section_1(){
   echo 'Text';
  }
  add_action('admin_menu', 'ach_faq_menu');
  </code></pre>
  
  <p><strong>How can I fix this ? Please help me !</strong></p>