It may be keyword relevancy.
I'm constantly adding new pages to several sites, and I've just been noticing the issue with some for the last week or two.
The odd thing is, that it won't display any ads, but if I click refresh a few (dozen lol) times, then it will go ahead and show the ads.
I'll go back and take a look at kw relevancy, because each pages is optimized for a specific keyword (in some cases might be multiple words).
I'm not really sure it's that though, because on one of my main sites, this isn't an issue at all, and although the content/articles are all thematically related, the language and H titles and all that are pretty disparate.