如果AdMob的奖励广告已经加载成功但是没有显示出来，可能是因为没有正确地将广告与显示的View关联起来。以下是一个可能的解决方法的代码示例：

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;
import com.google.android.gms.ads.rewarded.RewardItem;
import com.google.android.gms.ads.rewarded.RewardedAd;
import com.google.android.gms.ads.rewarded.RewardedAdLoadCallback;

public class MainActivity extends AppCompatActivity implements RewardedVideoAdListener {

    private RewardedVideoAd rewardedVideoAd;

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

        MobileAds.initialize(this, "YOUR_ADMOB_APP_ID");

        rewardedVideoAd = MobileAds.getRewardedVideoAdInstance(this);
        rewardedVideoAd.setRewardedVideoAdListener(this);

        loadRewardedVideoAd();
    }

    private void loadRewardedVideoAd() {
        rewardedVideoAd.loadAd("YOUR_REWARDED_AD_UNIT_ID",
                new AdRequest.Builder().build());
    }

    // ...

    @Override
    public void onRewardedVideoAdLoaded() {
        if (rewardedVideoAd.isLoaded()) {
            rewardedVideoAd.show();
        }
    }

    // ...

    @Override
    public void onRewarded(RewardItem rewardItem) {
        // 处理奖励
    }

    // ...
}

请确保替换代码中的YOUR_ADMOB_APP_ID和YOUR_REWARDED_AD_UNIT_ID为您自己的AdMob应用ID和奖励广告单元ID。此外，还要确保在布局文件中添加一个用于显示广告的AdView。