Admob奖励视频未在Android应用中加载

Question

这是我的主要活动代码：最近2天我都尝试了所有操作，但无法正常工作。请帮我。我已经尝试了AdMob的youtube教程和官方教程，但没有用。手机有问题吗？我为github尝试了一个示例项目，但是没有用，也许是因为它是一个非常老的SDK版本。随便..请看看开发人员。

package easyload.android.el.com.easyloadpk;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.MobileAds;
import com.google.android.gms.ads.reward.RewardItem;
import com.google.android.gms.ads.reward.RewardedVideoAd;
import com.google.android.gms.ads.reward.RewardedVideoAdListener;

public class OurSurvey extends AppCompatActivity implements RewardedVideoAdListener{
private RewardedVideoAd rvd;

    Button btn;
    private AdView mAdView;
    TextView txt;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_our_survey);

        btn = (Button) findViewById(R.id.btnad);
        mAdView = findViewById(R.id.adView);
        txt = (TextView) findViewById(R.id.textView);

     
       MobileAds.initialize(this,"ca-app-pub-3940256099942544~3347511713");
           rvd = MobileAds.getRewardedVideoAdInstance(this);
rvd.setRewardedVideoAdListener(this);
      //................................................................................
           loadad();
      

btn.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {


        if(rvd.isLoaded()){
            rvd.show();
        }
        else{
           // txt.setText("cnt");
        }

    }
});
    }


    private void loadad(){

        rvd.loadAd("ca-app-pub-3940256099942544/5224354917",
                new AdRequest.Builder().build());

        if(!rvd.isLoaded()){

            txt.setText("cnt");
        }
        else
        {
            txt.setText("loaded");
        }
    }

    @Override
    public void onRewardedVideoAdLoaded() {
        txt.setText("holalalala");
    }

    @Override
    public void onRewardedVideoAdOpened() {

    }

    @Override
    public void onRewardedVideoStarted() {

    }

    @Override
    public void onRewardedVideoAdClosed() {

    }

    @Override
    public void onRewarded(RewardItem rewardItem) {

    }

    @Override
    public void onRewardedVideoAdLeftApplication() {

    }

    @Override
    public void onRewardedVideoAdFailedToLoad(int i) {

    }

    @Override
    public void onRewardedVideoCompleted() {

    }
    @Override
    public void onResume() {
        rvd.resume(this);
        super.onResume();
    }

    @Override
    public void onPause() {
        rvd.pause(this);
        super.onPause();
    }



}

和此处的依赖项

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:27.1.1'
    implementation 'com.android.support:customtabs:27.1.1'
    implementation 'com.android.support:support-v4:27.1.1'
    implementation 'com.android.support:support-media-compat:27.1.1'
    implementation 'com.android.support.constraint:constraint-layout:1.1.2'
    implementation 'com.google.android.gms:play-services-ads:15.0.1'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
}