Have you added the 'submenu' => 'Something' parameter to your call to wp_nav_menus?
Your plugin works great for Posts or Custom links added to a WP menu, but does not work for Pages added to menus.
To fix…
Line 42 in submenu-filter.php is originally:
$item->slug = sanitize_title_with_dashes($item->post_title);
When you add pages to a WP menu, they don’t have a post_title property.
Simply change to use the ‘title’ property since all object types added to a WP menu have this property.
New Line 42:
$item->slug = sanitize_title_with_dashes($item->title);
I actually found and fixed the same problem only a few days ago, and have yet to publish a new version.
I’ve published version 1.2 which should fix this.
