Microsoft PubCenter change requires developers to update their Ad Units.
Microsoft PubCenter has recently introduced targeting changes aimed at improving application/ad relevancy.
If you use PubCenter ads in your Windows 8 or Windows Phone 8 apps, make sure to update your Ad Units to reflect the store category of the app that the ad unit is used in.
Three Required Steps:
1. Associate your Mobile ad unit to a specific Application via the pubCenter’s setup tab.
2. Select Category and Subcategory (when available) for each Mobile ad unit. Your selection should match the same categories you’ve selected in Windows Phone Dev Center.
3. Save, and repeat for each Mobile ad unit.
You will need to update all your existing Windows Phone Ad units by September 12st, to continue receiving ads. Ad units not updated by Sep 12, 2013, will stop serving ads.
More details can be found in the blog post in the Advertising Community Blog.