Here are some factors that might be causing the issue.
- Verify that you're using the correct account credentials for the ad source
- In addition, check with the ad source to confirm that your credentials has been approved.
- Verify that the country, devices, & cap targeting is set properly.
- It could be a fill issue based on ad source.
- You might want to double check where you have the mediated ad source prioritized in your waterfall - see ad source prioritization