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 Default campaign, which does not pay
- The visitor IP reached Paid Views Per Day
- Cookies or JavaScript are disabled
- Ad blocker or proxy detection
- The IP address changed
- A blocked referer
- Earnings are disabled
- A paid-view limit in the plan
In Campaigns mode, a country may have no matching campaign. To fix this, create or activate a non-default campaign for that country or for Worldwide (All countries). It must have inventory for the same traffic source.