Sorry you're having trouble! There could be multiple reasons why you're not seeing ads, including these.
Ad Network Status
Please make sure that the ad network, or ad source, that you have added is active and returning ads. You may need to check the status within their UI or contact your account manager. Some ad networks have a "test" mode that you can enable to ensure ads are always returned.
Ad Network Fill Rate
The ad network you're working with may not deliver an ad for each request, or in other words, they may not have a 100% fill rate. Therefore, you may not see an ad each time an ad request is made.
A frequency cap limits the number of ads that a unique person, or device, can view within a given time frame. For example, a common frequency cap is 3 per 24, which means that a given device or user would only be served 3 ads per 24 hours. If you are not seeing an ad, it is possible that a frequency cap for the ad source, or PLC, has already been reached.
It is very common for ad campaigns to have targeting parameters associated with them. Some common targeting includes GEO, device, and operating system (OS). If you don't see an ad for a particular ad source it could be due to targeting.
For example, if you are located in India and testing the ad source you may not see ads if the ad source has GEO targeting specific to the US, or other country.