There is a thread at the Microsoft Advertising Forum where one advertiser is complaining about his ads not being approved in the adCenter console. The issue is not his but rather a delay in the ad approval system.
Ricky Poole, Microsoft adCenter representative, said:
Thanks for reaching out. We apologize for the delay, we have been experiencing some system delays in our approval system. I have gone in and approved your ads, and everything is now live. Please don't hesitate to let us know if you have any additional issues in the future.
The advertiser said their ads were in review for a week or so with no status update.
So if you have the same issue, post in the Microsoft Advertising Forum.
Forum discussion at Microsoft Advertising Forum.