Immersive ads are barely showing? Am I doing something wrong?

About almost a month ago at this point I introduced immersive ads into my game Starving Sculptors.
After about 30 days here are the results.

More often than not the ad billboards are all completely empty. In fact I have never seen even one with an ad yet. This game averages around 150 ccu and can get up to 300 on the weekends. I have an obby that has only 10 players on a good day that almost performs on the same level in terms of ad income.

Am I doing something wrong with the implementation? Here is what it looks like in-game:

The only difference between my other game and this one is that I implemented custom billboards to display the ads.