Please, I have written many hubs and I also have a google adsense account but I am not earning. why?

One thing to be aware of is the type of things you write about. I took a quick look at your account, and it appears that most things you right about aren't very commercial. Commercial topics have much higher paying ads. For example, if you right niche product reviews like James.

http://hubpages.com/profile/James+Montgomery

His 50 or so Hubs have been making a few hundred dollars a month for over two years.

You only earn on the amount of clicks you get, not impressions. No clicks means your traffic isn't clicking on your ads. A good thing to bank on is 1 click every 60-100 page views (some get a lot more, some less, depends on content and audience).

Hope that explains it,

~thranax~