incrediBILL - 11:18 pm on Oct 21, 2012 (gmt 0)

Um, what does any of this have to do with what the OP was talking about?

Back to the topic at hand which is a site suddenly not showing ads on half the pages.

There has to be a global reason IMO and perhaps it's something in the site template causing it and only half the pages have been disabled because they were the ones indexed so far.

If you have a widget or something included on every page that includes external content, such as an RSS or twitter feed, that content could be temporarily adding stop words to the site as it's being indexed and they may resolve themselves when that content changes and the pages are reindexed.

Also, I've seen ads stop when pages couldn't be indexed, have you checked Google WMTs to make sure your site is being crawled properly?