i think you use a keyword that similar to your blog..?

I always thought that the ads are dependant on what you are searching for recently on Google. For example my wife was searching for Crocs for her father for xmas. Then when I went back on to do some site work thats all the ads were, ie crocs.

Eventually it does go back to the main site theme or keyword depending.

I have seen this issue in many websites or blogs and no one can't solve it 100%. Inserting provided codes by 'Peter Lee' can't help you too much too. Try it but i think it will not solve your problem. Showing ads in totally related to Google and it depends on Google Adsense which decides to display which ads for a page. Google Adsesne tries to shows relevant ads in each page but sometimes and for some websites ads are not relevant.
The only suggestion i can say it to insert some of your targeted keywords in the near of the place you are going to insert Adsense code.

Google uses your SEO content to determine which ads to give you. If their spiders can crawl your page and see that you're targeting the keyword "vacuum cleaners", they'll give you ads that relate to vacuum cleaners. The trouble is, if Google can't read your content and figure out what you're trying to target, you can (and likely will) get stuck with ads that have nothing to do with your site, and, thus, get lower CTR.

To get ads that are relevant, make sure all of your on-site optimization is taken care of properly - titles, tags, etc. Then, make sure all of your on-site content is properly optimized with the right keywords. Going back to the "vacuum cleaners" example, write a blog post like "5 Tips for Picking the Best Vacuum Cleaners" or "10 Ways to Get a Longer Life Out of Your Vacuum Cleaner".