admob视频广告未启动

import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.AdView;
import com.google.android.gms.ads.MobileAds;
import com.google.android.gms.ads.reward.RewardedVideoAd;
import com.google.android.gms.ads.reward.RewardedVideoAdListener;

public class MainActivity extends AppCompatActivity implements RewardedVideoAdListener {

    private RewardedVideoAd mRewardedVideoAd;

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

        // Initialize Mobile Ads
        MobileAds.initialize(this, "YOUR_ADMOB_APP_ID");

        // Create an instance of RewardedVideoAd
        mRewardedVideoAd = MobileAds.getRewardedVideoAdInstance(this);
        mRewardedVideoAd.setRewardedVideoAdListener(this);

        // Load a rewarded video ad
        loadRewardedVideoAd();
    }

    private void loadRewardedVideoAd() {
        AdRequest adRequest = new AdRequest.Builder().build();
        mRewardedVideoAd.loadAd("YOUR_REWARDED_VIDEO_AD_UNIT_ID", adRequest);
    }

    @Override
    public void onRewardedVideoAdLoaded() {
        // Ad loaded, you can now show it
        if (mRewardedVideoAd.isLoaded()) {
            mRewardedVideoAd.show();
        }
    }

    // Implement other RewardedVideoAdListener methods...
}