Ads On the Bottom of the Page

Basically, AdSense ads that are put at the bottom of the page just don't work well. When you read a web page, you read from the top to the bottom, and many people will never actually reach the bottom of the page, therefore never seeing your ad. Another reason that visitors may never see these bottom ads is if they are using a different size screen than you intended, and they simply don't scroll down to the bottom of the page.