Are you not seeing the placeholders themselves when you activate them, or are you not seeing any ads show within the placeholders?
The beauty of the Ezoic A.I and how it differs from an ad network is that you can cater for every user who visits your site and show them a personalized ad combination to suit their preferences - which may be very different from mine or yours for example. When you create placeholders with Ezoic, these are potential (temporary) ad locations. Ads won't display in all these locations at once unless the system has gathered data from the user that demonstrates this is their individual preference. So when you are seeing an ad combination which isn't to your taste, this is likely because as a web owner your behavior is atypical of how you'd browse as a visitor, and the system has categorized you differently because of that.