There are two threads on the topic of a possible bug with Microsoft adCenter. It seems like some advertisers are reporting seeing a major drop in the impression reports displayed for their search ads. A Microsoft Advertising Forum thread and a WebmasterWorld thread have reports from two advertisers.
Here is one report:
In the last few days several of our adcenter accounts have suddenly stopped generating spend. These are accounts with hundreds of adgroups and thousands of keywords. It's a sudden issue, but I suspect its somehow related to the migration. My reps have said they have other clients facing similar issues, but I can't find anything on the forums about it.
My server logs show short bursts of traffic for maybe an hour in the early morning and then not a single impression the rest of the day. Bids are not the issue, as I've tested raising some bids to 400% of their current levels and the ads still aren't showing.
In the Microsoft Advertising Forum thread, a Microsoft representative did reply telling the person to call in. The person did and was given information stating this is a bug, but the forum rep said it likely isn't a bug. Let me quote them.
Just got off the phone with a support engineer - this is a known system-side bug impacting a subset of advertisers. Starting this week, and occurring "randomly and intermittently", some accounts had their Ad Distribution setting set to "Not set", basically pausing all ad groups and campaigns in the account. This can be seen in the Medium column on the Ads tab. Unfortunately manually setting ad distribution to Search Network or other does not fix the problem. No known estimate to fix this...sigh...
Microsoft's forum representative replies:
There may be another reason as to why you are not receiving impressions or clicks in your account. When the Ad Distribution dispalys "Not Set" it means the distribution has not been determined as search or content yet because you have not received impressons in the adgroup. The Ad distribution in no way causes your adsto not show, there will be an underlying reason as to why this is occuring.
I understand our support team may have provided misguided information previously but I will have to recommend that you contact them again to have them look more in depth as to why your account is not performing.
So who is right? Microsoft or Microsoft? Oh wait?!
Of course this is a bigger issue now, cause Microsoft gobbled up Yahoo and now powers about 30% of the search ads in the US.