Hmm that does sound Odd, looking at your XAML it seems fine.
Note you only need to configure your AdProvider ID's in either the XAML OR the config XML, not both (we suggest in the config XAML as it's easier to maintain, especially if you host it on a free hosting site like Azure websites or AppHarbour)
Can you confirm you are calling the "invalidate" function for AdRotator in your code behind Loaded event (i'm guessing you are since it worked inside the pivot control)
one suggestion is to add a handler to the AdRotator log event and output it's messages to the DEBUG window to see why it's disabling, usually this is because there are either No ads to serve or the configuration is wrong.
AdRotator.Log += (s,e) =>
(p.s that's just an example off the top of my head so may not be exact)
Other option is to put the AdRotator control in a separate user control and then use that user control in your page (making sure to call invalidate in the user controls loaded event in the code behind), we usually recommend this as it's much cleaner
especially if there are multiple places where you use the control.
Hope this helps.