First discover never show automatically in the tray that upgrades are available. It seems automatic upgrades discovery does not work. If I do an apt-get update on a console and updates are available, then : - immediately the discover icon appear in the tray showing the blue dot for upgrades available - if I clik on this icon discover opens, then shows NO UPGRADES ARE AVAILABLE - if I click on the discover menu the "check upgrade" button then discover show briefly that upgrades are available then shows that NO UPGRADES ARE AVAILABLE - after some time I will finally succeed in seeing the list of available upgrades in discover gui quite not coherent behaviour :)
*** Bug 372838 has been marked as a duplicate of this bug. ***
*** Bug 373269 has been marked as a duplicate of this bug. ***
This was fixed some time ago