This is a common question for people who are using Google AdSense, and there are a variety of reasons why you might find that it's not targeting good ads on your site. Let's talk about the major causes and I'll explain how to remedy each of them.

The most common cause is that your keyword density is too low. You might not have heard of this SEO jargon, but the logic is straightforward: if you have a blog about knife self-defense, for example, you need to actually have the phrase knife self-defense on your site at least once every 2-3 paragraphs.

All too commonly, writers find that they refer to "it" or, in your case, "self defense" or "protection" or other synonyms. A synonym is a good tool for writing, but it can be counterproductive for having search engines or ad engines accurately identify the primary topic of your article.

Another facet of this is if you have lots and lots of non-article material on your pages (promotional text, ad copy, link lists) you can confuse the ad engine about just what actually is the point of the page. This can best be addressed by adding AdSense section targeting, as explained here: AdSense Section Targeting.

A second problem you might be encountering is that there are simply too few bids on your exact keywords in the AdSense system (technically, AdWords, as that's the "advertiser" side of the AdSense coin). You can ascertain this by opening up an AdWords account and bidding on a few of the most important keywords in your space. Does a bid of $0.05 give you top placement? That means that there aren't any other advertisers!

Another way to test is to use a tool like Nichebot. When I go there and search for "knife self-defense" shows that it was only searched for 15 times in the last 12 months (in their database, at least). That's incredibly rare, and strongly suggests that you need to come up with either a broader focus for your blog (by comparison, "self defense" was sought 782,000 times) or come up with a different description of your content to be more findable.

Finally, if you do these and use these techniques and still find that your ads aren't a good match, you might email Google's AdSense support team and ask them for some suggestions. I've found them friendly, helpful and reasonably responsive, and it is in their best interest to be matching your content as closely as possible.

Hope that helps you get up and running with better ads.

Oh, and if you are just beginning with Google AdSense, can I suggest that there's no better place to get started on the Web than here: Get Started with Google AdSense.