The ads that show on the site must be on topic for anyone to click on them, the visitor to your site is there for that certain subject your site covers. I have found that by changing the type of content on a page and altering the keyword density significantly alters the ads showing on that page. It seems that underlining and bolding keywords (but only once or twice) and correct placement of words on the page also makes a difference. If your site does not show the ads you want you must look at the set out of the content and remember how the engine will read it. This alone can make the difference between on topic ads showing and either non topic or even no ads at all.