if ads related to your content is not available it shows the next best ads, so does happen, load up more content to your topics tht should help
The ads are based off content in your site. They use key words in your text. However, sometimes Google pulls out a keyword and places ads that has nothing to do with the general topic of your site. I have a political blog and I wrote a post that included the word gay and for weeks all I got was ads for gay rights and gay sites.