If you are testing immediately after creating a live campaign/flight, please wait at least 15-20 minutes before testing. It is normal to not hear ads for the first few downloads. For details, see Your Podcast Might Have Low Download Numbers.
If, during initial testing before the ad creative is in our cache, you do not allow the file to play all the way through and test again within a 24 hour window, you also are unlikely to get ads based on how our ad server sessions are held.
Note: This blue waveform is the raw, originally uploaded audio and does not have any post-processing or ads.
You can test that your campaign is running by playing the preview embed as below - this is what your listener is delivered and includes post-processing and ads (if they're applied).
Or you can use the redirect debugger here to confirm that the download is being redirected to your ad-server for dynamic ad insertion.
Redirect Debugger
You can use the redirect debugger here to confirm that the clip is being redirected to your ad-server correctly:
If your download is being redirected, then you will get this green tick:
To understand how downloads and ad impressions can and will differ, please see this explainer.