This issues may raised below mentioned reason :
1 . You may set the any targeting limitation for your banners and that limitation not matched for your criteria.
2 . You may set campaign or banner session capping and that capping was completed for your current session.
3 . You may not linked any active campaigns with active banners for that current zone.
4 . Your campaign or else banners not in active status for that current zone.
5 . May be your banner wight was ZERO . We should need at least weight = 1 .
6 . This may be raised due to OpenX cache files .Delete your all cache files under “YourOpenx/var/cache” .
The above mentioned criteria not fixed your issue then let us know your query via ” Contact Us “ page in our site .