You know what your blog is about, the theme/ topic you write about, the niche you stick to. Think about who you are writing for/ to (your readership). Your readers are looking for niche/ topical content when they come to your site. Do the ads on your site fit in?
Turn that around, consider the sites you run ads on or exchange links with. Do you run ads based on traffic and stats alone? Are you wasting your time and resources looking for readers in the wrong places?
Will the people who read that site see your ad and be interested in visiting you? What does your site have in common with the site your ad appears on?
You don’t have to stick to sites in your same genre or niche. However, running ads too far out of your niche like sports, financial or political sites if your site is about creative arts, selling your crafts, isn’t as effective as choosing other creative sites. Instead of a football fan site, try sites by writers, photographers or knitters. Look for inspirational or coaching sites if you write in a creative niche. People who create are always in search of fresh inspiration. Look for sites about using resources, saving money and being green in profitable ways if you run a site about business and related topics.
The site your ad runs in should compliment the niche of your own site. A blanket ad campaign will find some readers for you but you could run the same ad campaign with some focus on where they appear and get results focused on what you actually have to offer.