Admob interstitial ads are not closing

Question

I'm showing interstitial ad in my app and ads are loading and displaying perfectly but the issue is that ads are not closing when I click cross button on back button I have to quit my app to close them I created a new project and checked the ads in new project they are working fine and closing on back button but in my main app ads are not closing once they displayed to user this is my Admob class

public class Admob {

    public static InterstitialAd mInterstitial; // Interstital
    private static AdView mAdView; // banner

    public static void createLoadInterstitial(final Context context, View view)

    {

        mInterstitial = new InterstitialAd(context);
        mInterstitial.setAdUnitId(context.getResources().getString(
                R.string.admob_showIntersitial_ad_unit_id));
        mInterstitial.setAdListener(new AdListener() {
            @Override
            public void onAdLoaded() {
                // TODO Auto-generated method stub
                showInterstitial();
            }

            @Override
            public void onAdFailedToLoad(int errorCode) {
                // TODO Auto-generated method stub

                super.onAdFailedToLoad(errorCode);
            }

            @Override
            public void onAdOpened() {
                // TODO Auto-generated method stub
                super.onAdOpened();

            }

            @Override
            public void onAdClosed() {
                // TODO Auto-generated method stub
                super.onAdClosed();

            }

            @Override
            public void onAdLeftApplication() {
                // TODO Auto-generated method stub
                // Called when an ad leaves the app (for example, to go to the
                // browser).

                super.onAdLeftApplication();
            }

        });

        loadInterstitial();

    }

    public static void loadInterstitial() {

        mInterstitial.loadAd(new AdRequest.Builder().
                addTestDevice("").//ca-app-pub-3940256099942544/1033173712
                build());
    }

    public static void showInterstitial() {
        if (mInterstitial.isLoaded()) {
            mInterstitial.show();
        }
    }

    public static void createLoadBanner(final Context context, View view) {
        mAdView = (AdView) view.findViewById(R.id.adView);
        AdRequest adRequest = new AdRequest.Builder().
                addTestDevice("").//ca-app-pub-3940256099942544/6300978111
                build();
        mAdView.loadAd(adRequest);

        mAdView.setAdListener(new AdListener() {

            @Override
            public void onAdLoaded() {
                // TODO Auto-generated method stub


                super.onAdLoaded();
            }

            @Override
            public void onAdClosed() {
                // TODO Auto-generated method stub

                super.onAdClosed();

            }

            @Override
            public void onAdOpened() {
                // TODO Auto-generated method stub

                super.onAdOpened();
            }

            @Override
            public void onAdLeftApplication() {
                // TODO Auto-generated method stub


                super.onAdLeftApplication();
            }

            @Override
            public void onAdFailedToLoad(int errorCode) {
                // TODO Auto-generated method stub


                super.onAdFailedToLoad(errorCode);
            }

        });

    }

}

This is how I show them in my activity

 MobileAds.initialize(this, "ca-app-pub-3940256099942544~3347511713");

        Admob.createLoadInterstitial(this,null);

I have initialize this in Mainfest also

  <meta-data
            android:name="com.google.android.gms.ads.APPLICATION_ID"
            android:value="ca-app-pub-3940256099942544~3347511713" />

I saw same questions on stackoverflow but none of them is answered the solution so any help will be really appreciated