Ad Mob Problem

I used the example showed on the coronalabs tutorial to implement admob

When i built the app and i tested it on my android everything was gone well. The ad had been showed but i didn’t clicked on it… then i closed the app and I reopened it… but the ad didn’t display anymore… i try to delete the app and reinstall but nothing…

whant’s the problem???

PS it is an interstitial ad not a banner