Why does menu items start disappearing In Drupal 7 some menu items, even though they exist in menu_links db table, do not appear in the menu. I wonder if it has something to do with cache, but I have disabled the cache and cleared it. 

If I add a menu item that goes for instance to &lt;front&gt;, it appears in the menu, but if I add a menu item for a certain node, it does not appear. I can't find the logical reason for it.
The only reason I can think of is that the viewing user does not have access to the linked node. Internal menu links that point to unauthorised content do not appear in menu.

Make sure the node has been published and your user can view it.

Some people actually consider dolbydolby's work around a bug :-) http://drupal.org/node/483150