may be 1. your ads is PSA. 2. your ads is impression based, not ppc. 3. your revenue only $0,0000000001, so its written $0 4. your click not from certain country.
Or you might be smart priced. If none of the above statements applies to you, try removing adsense ads for 30-40 days, use an other publisher meanwhile, and put them back later on. And do a search on google about smart pricing, then try to find a solution for your website.
Is not your little click on CPC and CPM, too little there is no income, I've also come across such a situation.