The Diagnostic Console keeps history only as long as it is open. When the console is closed all information is lost.
Workaround:
In the Spotlight on Messaging MMC console | Web Reports you can view historic information from your health tests.
For more information on the Web Reports or configuring health tests consult the Spotlight on Messaging user guide.
C:\Program Files\Quest Software\Spotlight on Messaging\Documentation\Spotlight_on_Messaging_User_Guide.pdf
