Did you select only 12 ad providers? Go to Admob, on the left menu go to Blocking controls. On the top sliding menu go all the way to the right to EU User Consent and Scroll down to "Custom set of ad technology providers". Click Select Providers and pick a few, no more than 12.
In your code in the Create action set "show" to FALSE, not true.
Create another event for testing. Create new Text object and add event:
-> MobileAdvert: On interstitial failed to load --> TextObject: Set text to MobileAdvert.ErrorMessage
You can do the same thing with On configuration failed to see if that is the problem.
Export for android and then test on a few different devices. I have the same app normally showing test ads on 2 devices but not on the 3rd one - no idea why.