admob interstitial Ads in sometime showing black screen

Sometimes the interstitial ad is displayed right, but other times it is display a black screen I don't understand what is the problem?

this my code:

MobileAds.initialize(this,
            getString(R.string.admob_app_id));

    mInterstitialAd = new InterstitialAd(this);
    mInterstitialAd.setAdUnitId(getString(R.string.admob_interstitial));
    AdRequest adRequest1 = new AdRequest.Builder()
            .build();
    mInterstitialAd.loadAd(adRequest1);
    mInterstitialAd.setAdListener(new com.google.android.gms.ads.AdListener() {
        @Override
        public void onAdLoaded() {
            mInterstitialAd.show();
            super.onAdLoaded();

        }
    });