Reports via WebmasterWorld are that Google's ad diagnostic tool will indeed not work properly in all cases at this moment.
The reported case is;
Searching for our ads via a few specific kw's, I notice they aren't showing. Diagnostic tool - both elements of it - state that there are no ads matching these search terms. Stats, this month and last, say otherwise (in fact they are long-standing kw's).
AdWordsAdvisor explains that there is an issue with the tool and it won't be fully fixed for several days.
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.
Forum discussion at WebmasterWorld.