If Google doesn't have enough information to determine what your site is about, they'll display Public Service Ads (PSA). These are basically ads that "help the world" but don't make you any money. If you're finding PSA ads on your site for more than 72 hours, then chances are something's not working properly with your AdSense set-up.
Here's how to ensure that PSA ads don't show up.
Reduce Your Ad Units
If you're noticing that some ads are showing actual advertisers while others are PSA ads, that probably means Google is able to tell what your site is about but just doesn't have enough ads to show.
Reduce the number of ads on your site. Perhaps run just one prominent ad rather than several ads at a time.
Add More Content
This will help Google determine more easily what your site is about. In fact, without at least a base amount of content, AdSense may not even be able to tell what your site is about.
Add more content. Make sure you have at least 500 words of content on each page, preferably 1,000 words or more.
Use the Google Keyword Tool to find these LSI keywords. Type your main keyword into the Google Keyword Tool, then see what kinds of keywords Google thinks are related to your site. Write down a few of these keywords.
Then go into Google search and type in your keyword. Scroll all the way to the bottom, where it says "Related Searches."
Write down all of those searches as well.
Basically what you've done is take your main keyword and find out what Google thinks is related to that keyword. No guessing necessary.
Now take those keywords and add them to your site in a way that makes sense. Don't just stuff them into your page, but weave them into your page whenever possible.
Add an Alternative Ad
Google allows you to specify an alternative ad instead of using their Public Service Ads.
In other words, you can tell Google to display an ad for an affiliate product or an ad from another ad network if they can't find any ads for you.
If you're finding PSA ads appearing often, this is a great way to recover some of that lost income.
To sum up, start by removing some ad units so Google needs to find fewer ads to fill up your ad space. Add more content. Use the Google Keyword Tool and Google Search to find LSI keywords and add that to your content. Finally, specify an alternative ad to recapture lost income.