It works like this, that each visit is inserted as a bot visit by default, and when the joomlawatch image is loaded (by browser), the system is able to get the visitor's user agent info. (Bots are usualy not loading images)
Then, this info is updated, and user is moved from bot section into visitors section.
So maybe there's something wrong with your img.php that it's unable to qualify user as a visitor.
Try to edit img.php and comment error_reporting(0), then try www.yoursite.com/components/com_joomlawatch/img.php
and post the result here