To identify the issue, enable debug mode in the player settings: Settings / Service / Log messages to the browser console
Then check the browser console Developer Tools → Console in Chrome
You will see player messages marked VAST, including the reason why ads are not shown — it may be empty or contain an error. In that case, contact your ad network for more details.
If there are no ad-related messages in the console, make sure the player is up to date. Then submit a bug report on the player page in the builder (click the beetle icon in the top menu). Include a link to the player page and a detailed description of the issue.