WebmasterWorld has reported that presently Google's ad diagnostic tool will not be functioning properly in all cases. In some cases when you search for ads with a few specific keywords, the diagnostic tool reports that there are no ads matching those search terms. Whereas the stats for last month as well as this month stated differently.
According to the AdWords Advisor, " There is currently an issue with the Ads Diagnostic Tool which will cause it to return the message
you are describing in at least some cases. Due to interdependencies with other stuff, it's likely to be several days at least before this is resolved."