People often ask me why their earnings are so low and there are many reasons for that.
1. Not enough traffic
If your site has less than 500 visitors per day, it's going to be difficult to make much with AdSense. You have to remember that only a small percentage of people will click on your ads (1-2%) so if you have 500 people coming to your site in a day, that's 5 clicks.
Also, most clicks will earn you less than $1, so hopefully you can see that it takes a great deal of traffic to build up income with AdSense.
2. Not enough relevant ads displaying
Some niches have more, relevant ads that Google can display. This will drastically impact your click through rate. If the ads aren't relevant to your content then your visitors aren't going to click.
That's why it's important to make sure your site has a lot of content so you can help Google display the most relevant ads. Granted, sometimes there aren't relevant ads available, and there's nothing you can really do about that.
However, just having a few sentences with some images is not helping Google find the best ads because you have so little content.
3. Low paying niche
Let's face it. Some niches pay more than others. If you have a finance site, the ads will probably pay more than the ads on an entertainment site.
So the niche you are in plays a big role in how much you can make. That's why it's hard to estimate how much anyone can earn with AdSense. There are just too many different variables.