'This is not spam' button perpetually grayed out (Outlook 2007)

For some reason the 'not spam' button is always inactive with NIS/Outlook 2007.  Is there a way to activate it?  Barring that, can I somehow mark messages so that they don't get flagged in the future?  I sometimes get 2 messages from the same sender, one gets flagged as spam and the other does not. 

 

Thanks!