title: AdMob Interstitial ad not showing on all my test devices
body: <p>Hello I have implemented interstitial ads in my android game, first of all while testing the ads I've noticed that they appear only once per game session,I'd like to know if it will behave this way when I update the game because I don't want people to be overwhelmed by ads, once per game is sufficient.Second, when I test the ad on my S3 Mini it works correctly,however when I test it on a Galaxy S plus is does not work. The ad works like this,when the game is over I change set the value of a static variable called showAd inside my settings class to true.Then in my activity class I have:</p>
  
  <pre><code>  @Override
  public void onUserInteraction(){
      if (Settings.showAd){
              displayInterstitial();
      }
  }
  </code></pre>
  
  <p>so that when the user presses the screen after the game is over an ad pops up. Is the onUserInteraction method unavailable on lower android versions ?Thank you! ( I have added the S plus as a test device in the AdRequest method, still doesn't work)</p>