Open Admin > Reports or Admin > Advanced > Statistics Table and check the recorded reason.
Common causes include no eligible active non-default campaign with remaining inventory, a non-paying Default campaign, the visitor IP reaching Paid Views Per Day, disabled cookies or JavaScript, adblock or proxy detection, an IP change, a blocked referer, disabled earnings, or a plan paid-view limit.
In Campaigns mode, country-coverage problems are normally resolved by creating or activating a non-default campaign for that country or Worldwide (All countries) with matching traffic-source inventory.