It depends on a few things. How many sites do you have adsense on? How did you implement your ads on your site? How many visitors are you getting daily. And of course, how much are your clicks earning you. Maybe if you show the site, we can help you. Leslie
Its not important how many sites. One good content base site can make good money with adsense. I have my own success story with Content base sites, there people love to click on relevant ads. Thanks.
There are many people doing a lot less than $20 a month on adsense so don't be too down. You could easily improve that though by reading up some of the many blogs about adsense optimization. Make sure they are positioned in the best possible place, using the best colours, etc.