Why are interstitial ads not showing on device

I developed an Android app and I integrated Google's adMob interstitial ad. The problem is that on the emulator the ad is successfully shown but on my device it is not showing up. I created the Ad Unit Id that I created on AdMob and I linked the App to adMob.

Here is My code :

InterstitialAd mInterstitialAd;

mInterstitialAd = new InterstitialAd(this);
        mInterstitialAd.setAdUnitId(getString(R.string.interstitial_full_screen));
        AdRequest adRequest = new AdRequest.Builder().build();
        // Load ads into Interstitial Ads
        mInterstitialAd.loadAd(adRequest);
        mInterstitialAd.setAdListener(new AdListener() {
            public void onAdLoaded() {
                showInterstitial();
            }
        });

Now the showInterstitial() function :

private void showInterstitial() {
    Random r = new Random();
    if (mInterstitialAd.isLoaded()) {
        new android.os.Handler().postDelayed(
                new Runnable() {
                    public void run() {

                        mInterstitialAd.show();
                        AdRequest adRequest = new AdRequest.Builder().build();
                        mInterstitialAd.loadAd(adRequest);
                    }
                },
                r.nextInt(7000 - 5000) + 5000);

    }
}

I added a random timeout and when the ad is loaded it will be displayed.

and In my gradle file I added the following :

compile 'com.google.android.gms:play-services-ads:9.8.0'

NB. I have another banner ad in the App.

on the emulator it works just fine here is a screenshot :

Any One have any idea why the Interstitial Ad isn't showing on devices? . Thank You.

I fixed the issue but changing how I call my showInterstitial() function . The problem was related to the loading of the ad. Here is my fix :

  @Override
    public void onResume() {
        // Start or resume the game.
        super.onResume();
        showInterstitial();
    }
    @Override
    protected void onStart() {
        super.onStart();
        showInterstitial();
    }

I show the Ad when the Activity Starts or when it is resumed , we need an event to show the Ad.

This is how I declared the interstitial Ad and my showInterstitial() function :

//Interstitial
            private InterstitialAd mInterstitialAd;
            mInterstitialAd = new InterstitialAd(this);
            mInterstitialAd.setAdUnitId(getString(R.string.banner_ad_unit_interstitial));
            AdRequest adRequestInterstitial = new AdRequest.Builder().addTestDevice("deviceid").build();
            mInterstitialAd.loadAd(adRequestInterstitial);

            mInterstitialAd.setAdListener(new AdListener() {
                @Override
                public void onAdClosed() {

                }

                @Override
                public void onAdLoaded() {
                    mAdIsLoading = false;
                    showInterstitial();
                }

                @Override
                public void onAdFailedToLoad(int errorCode) {
                    mAdIsLoading = false;
                }
            });

This is my Result on my Samsung Device :

I Think that was the issue.