Please be specific to the point. Let us know URL of your site, what niche you're in and what kind of ads showing. Please note that, there are several reasons behind your issue. In most cases your content and keyword target matters, if you don't have a authority content, or proper keyword target Google ignores and just displays the lowest CPC or low rated ads.
If there are no advertiser on keyword, google will search for the nearest niche relevant advertiser for your website. Also, If a user enters your site on different keywords/query the ads will be based on the keywords/query of the user.
In such cases I always suggest the first thing to do is up the keyword density - 9 times out of 10 this is the problem. But it's down to you not providing enough keywords, not google failing in any way. "A map is useless you tell someone where they need to go to".