请求后未加载广告

我想在加载广告时显示一个按钮,然后单击按钮显示广告。

我的问题是,广告从未加载。

这是我目前的代码:

 MobileAds.initialize(getActivity(), "ca-app-pub-***");
        mInterstitialAd = new InterstitialAd(getActivity());
        mInterstitialAd.setAdUnitId("ca-app-pub-ca-app-pub-***");
        mInterstitialAd.loadAd(new AdRequest.Builder().build());

        btnOfertar.setVisibility(View.GONE);


        mInterstitialAd.setAdListener(new AdListener() {
            @Override
            public void onAdLoaded() {
                // Code to be executed when an ad finishes loading.
                Log.i("Ads", "onAdLoaded");
                btnOfertar.setVisibility(View.VISIBLE);
            }
        });