Verify your MMP settings and, most importantly, that attribution works.
Double-check your campaign configuration
Before launching an iOS campaign, ensure that the advertiser app is configured correctly with SKAdNetwork according to the documentation from Apple.
Update your Unity Ads SDK to the latest version if you have not done so already. (Android and iOS)
For launching an iOS ROAS or retention campaign, follow the Unity knowledge base article on Audience Pinpointer.