Sometimes this happens because the visitor waits very long before clicking on your site. A visit can be registered on day, and the clicked on the next day. Say that the visitor comes to your site in the evening, and then leaves the computer. After getting back (maybe the next day), your site is still loaded but the visitor session has ended a long time ago according to Google (since there has been no activity in a long time). So now the visitor clicks on your site. Google see the click, but no ad impression. Since it occured yesterday. Anyway, you should try to get some more traffic
Not every click is worth something Maybe they clicked on an CPM ad? Maybe it's such a bad keyword + smart priced to being under half a penny? You've still got revenue from it if it's showing in your Today section