InterstitialAd Admob not visible

// on bottom of onCreate add listener
mInterstitialAd .setAdListener(new AdListener(){
     public void onAdLoaded(){
          interstitial.show();
     }
});

} // end onCreate

 public void displayInterstitial (){
        if (mInterstitialAd.isLoaded()) {
            mInterstitialAd.show();
        }
    }