This behavior is encountered because some users made some settings to the notification feature. To change this, the users with problems should press the little arrow next to the system clock and then choose Customize (see the picture below). In the new window that opened, search for this program and from the drop down menu that is next to it, chose Show icons and notifications. After you did this press OK and close the window.
http://i61.tinypic.com/10mnf5z.jpg