A Digital Out Of Home (DOOH) billboard might reject an ad for several reasons, including:

1. Content Violations: Ads that contain explicit, offensive, or inappropriate content, such as violence, adult themes, or hate speech.
2. Non-Compliance with Regulations: Ads that do not comply with local advertising regulations or industry standards, such as restrictions on political messages, alcohol, or tobacco advertising.
3. Brand Safety Concerns: Ads that could negatively impact the image or reputation of the billboard owner or the brands they represent.
4. Technical Issues: Ads that do not meet technical specifications, such as incorrect dimensions, low resolution, or incompatible file formats.
5. Intellectual Property Violations: Ads that infringe on copyrights, trademarks, or use images or logos without proper permissions.
6. Conflicting Advertisers: Ads that conflict with existing advertising contracts, such as promoting a competitor's product.
7. Inappropriate Targeting: Ads that are not relevant to the location, audience, or context of the billboard.

These are some of the primary reasons a DOOH billboard might reject an advertisement.