Some system tray icons invisible in Gnome Classic 12.04
The solution is to add the Notification Area applet.
Taken from this question No Banshee notification area icon in Gnome 3 classic session
I have just solved this problem with davMail on xubuntu using the following method:-
- Open the task manager and if davmail is running, kill it.
- Open a terminal and type
sudo apt-get install libswt-gtk. - Re-start DavMail and you should see the icon in the notification area.
It took me along time to find the solution, but it is quick and simple to do.