任何调解广告网络都不会为admob测试广告显示任何内容

Question

我正在尝试在我的应用上投放测试广告，但我总是收到此错误

06-14 19:42:04.223 7573-7573/com.pk.kapp I/Ads: Starting ad request.
06-14 19:42:08.757 7573-8262/com.pk.kapp I/Ads: Trying mediation network: 
06-14 19:42:08.767 7573-7573/com.pk.kapp I/Ads: Instantiating mediation adapter: com.google.DummyAdapter
06-14 19:42:08.767 7573-8262/com.pk.kapp I/Ads: No fill from any mediation ad networks.
06-14 19:42:08.777 7573-7573/com.pk.kapp W/Ads: Failed to load ad: 3

我在片段的onCreate中执行此操作：

@Override
public void onCreate(Bundle savedInstanceState)
{
    Log.d(TAG, "onCreate");
    super.onCreate(savedInstanceState);

    mInterstitialAd = new InterstitialAd(getActivity());
    mInterstitialAd.setAdUnitId("ca-app-pub-3940256099942544/1033173712");
    AdRequest adRequest = new AdRequest.Builder()
            .build();
    mInterstitialAd.loadAd(adRequest);
    mInterstitialAd.setAdListener(new AdListener() {
        @Override
        public void onAdClosed() {
            pageContent.setVisibility(View.VISIBLE);
        }

        @Override
        public void onAdFailedToLoad(int i) {
            Log.d(TAG, "Ad failed to loadvand error code is " + i);
        }

        @Override
        public void onAdLeftApplication() {
        }

        @Override
        public void onAdOpened() {
        }

        @Override
        public void onAdLoaded() {
            Log.d(TAG, "Ad has has loaded to load");
        }
    });
}

拜托，请问有什么解决方法吗？喜欢它的原因以及如何解决它。