I read the hub I believe you are talking about. The little girl, soldier, shiny boots one? I have no idea why the ads aren't showing up on it.
A thought, though: go back in to edit mode on your hub and make sure this box is checked: "content meets the conditions of the Google AdSense Program Policy." If it's not, I don't believe the ads will display.
There could be another reason if it's not that. Try contacting the hub team, or post a message in the forum.