It's possible that you've defined a narrow target. If a target is too specific, there might not be enough available ad space and demand within the target to place an ad.
If you use geographic and other targeting settings, but the settings are too narrow, the platform automatically adjusts to use geographic settings only. This helps ensure that a target is wide enough to meet placement goals.
If you only use geographic targeting settings, but the settings are too narrow, the platform does not automatically adjust to broaden the target. One solution is to specify broader geographic settings.