I have checked your issue and found that you are trying to load the ads at the top of the page and you have also enabled the viewability option, if you want to show the ads in the top that is after the header etc that is 600px top from the viewport then the amp-iframe will not load.

So the simple solution is to disable the viewability option from advertisement and your issue will get fixed and all your ads will work perfectly – https://prnt.sc/lurt76.