You need to have tightly focused content, and lots of it, throughout your site. Although Google does serve ads based on the specific content of a single page, it uses its knowledge of your entire site to help determine the content.

You'll want to make sure you use your keywords a few times, and different various of your keywords, on the page you are working on.

Also, you may need to give it a couple days. If Google spiders your page and can't figure out relevant ads to serve, sometimes you'll get the PSA for a couple days before it comes back for another look, even if you tweak the content.