One of the reasons is the number of blogs you have and the positioning of the adsense in the blogs. You have to realise that in configuring the adsense you have to chose the 160x600 wide banner and place it at the right hand side, that is the side bar of your blog. For instance i run well over ten blogs in different niches, in all of them, the adsense placement is as described above.
one of them include
Now, notice that each blog is highly targetted to a particular niche, for instance the one on how to lose weight normally brings out adverts in the adsense on how to lose weight forcing most of the visitors who are looking for ways to lose weight to click on the ads. Hope you catch the gist well, likewise the rest in other niches, the adsense throws up adverts on what the visitors will be tempted to click. There is no magic to it, just write relevant posts to your blog theme and the ads will appear naturally.
Another way to earn from blogs heavily is to integrate affiliate marketing into it. Having created blogs in multiple niches, apart from adsense, look for an affilite program that will match that niche and paste the banners in your blog, like if you visit any of the blogs i listed above including others i did not mention, you will see affiliate banners that relates to a particular blog there. By virtue of having more than ten blogs in highly targetted niches, i also have affiliate products in all of them. One of the ways i use to send targetted traffic to each of the blogs is through yahoo answers. I answer questions that partains to a particular niche in yahoo answers and leave the blog address in the resource link that appears below and sometimes integrating the blog address inside the answer itself, all the niches i have blog on have relevant questions at yahoo answers.