[@netweb Keymaster 12 years, 4 months ago]

They could be getting caught by Akismet or are failing the WordPress Comment Moderation/Blacklist limits are have an assigned status ‘pending’

http://example.com/wp-admin/edit.php?post_status=pending&post_type=topic
http://example.com/wp-admin/edit.php?post_status=pending&post_type=reply