There are confirmed reports at Google AdWords Help forums of a bug with the Google AdWords Ad Preview and Diagnosis tools.
Some are reporting that when testing the localization within the tool, the tool is showing that the ads are not running, when they are indeed running.
Dan S, an AdWords advertiser wrote:
I'm having a similar problem with the Ad Preview Tool but the ads themselves seem to still be running. Keyword Diagnosis gives same result as bubble and preview tool.
Google's Kathleen confirmed the issue and said the Google team is looking into it. Kathleen wrote:
Thanks for bringing this to our attention. This issue has been detected by our technical team and they are currently looking into this. I'll update this thread when I have more information!
Forum discussion at Google AdWords Help.

Comments:
PPCPROZ
09/15/2014 12:32 pm
In addition, the shopping preview, usually showing on page two, is not showing now. when will shopping be previewable from page 1, and on tablet/smart?
Mark Anthony
09/15/2014 01:54 pm
I'm getting the exact same bug in ad preview
Pavan Rai
09/16/2014 08:41 am
Yes..That's right. I am also facing same bug.
Wawka
09/16/2014 09:06 am
When will the preview tool actually start reflecting the way the SERPS look - e.g. inclusion of PLAs which have a huge impact on ad placement.
Michael Climer
09/16/2014 03:00 pm
Seeing this issue over the past days myself. Making life harder than it needs to be thats for sure. I notice if I change the targeting setting from people in my included locations to people in, searching for or viewing pages about, the ads will show in the Preview Tool. Just an observation.