Check the page on your site that brings in zero ... and see if you don't have Public Service Ads showing... they pay nothing if someone clicks on them... PSA appear if you don't have enought content on the page or if your keywords can't generate any "proper" ads...
It could also be a CPM ad or the stats just not updating in real time (i.e. clicks updating before revenue).
Google don't pay if you accidentally click on your ads. Or if someone click two times on the same ad.