I recently updated my website after a while, and the custom fields meta box is not showing in the editor anymore. It isn't showing under "Screen Options" either. Any ideas why this could be, and how to get it back?
When registering a custom post type you have to declare that it supports the custom fields meta box to get it, e.g.:
'supports' => array( 'title', 'editor', 'custom-fields' )
But most people don't do this, and build real metaboxes instead. This way instead they can put in radio buttons and drop downs etc