About two weeks ago, an advertiser reported a bug with Google's Ad Preview and Diagnosis tool in the Google AdWords Help forums.
The issue was that when filtering to mobile, it wouldn't show ads based on the location the advertiser set. For example, the advertiser is in Toronto, Canada but wanted to see ads as they would appear in Miami, Florida. But it kept only showing the advertisers ads in Toronto.
After weeks of trying to debug the issue, thinking it was user error, Google's AdWords representative Mini updated the thread saying it was a bug on Google's end. She wrote:
AdWords engineers have identified this as a technical issue. They are now working to fix this. We will keep you posted on any updates.
So if you use this tool, beware it might have some issues showing you legit ads.
Forum discussion at Google AdWords Help.