Admob rewarded video will only appear once

Question

I need to assemble Admob in an android program packaged by Cocos creator. When I click the "watch video" button, the rewarded video plays fine. But if I "close video" then try to "watch video" again, it does not work. When I debug the program, I find this code can not run again.

new RewardedAdLoadCallback() {
    @Override
    public void onAdFailedToLoad(@NonNull LoadAdError loadAdError) {
    // Handle the error.
    Log.d(TAG, loadAdError.toString());
    rewardedAd = null;
}

@Override
public void onAdLoaded(@NonNull RewardedAd ad) {
    MyApplication.this.rewardedAd = ad;
    Log.d(TAG, "Ad was loaded.");
}

My loadAwardAd code is:

public void loadAwardAd() {
    AdRequest adRequest = new AdRequest.Builder().build();
    RewardedAd.load(this,
        "ca-app-pub-3940256099942544/5224354917",
        adRequest,
        new RewardedAdLoadCallback() {
            @Override
            public void onAdFailedToLoad(@NonNull LoadAdError loadAdError) {
                // Handle the error.
                Log.d(TAG, loadAdError.toString());
                rewardedAd = null;
            }

            @Override
            public void onAdLoaded(@NonNull RewardedAd ad) {
                MyApplication.this.rewardedAd = ad;
                Log.d(TAG, "Ad was loaded.");
            }
    });
}

I reload the loadAwardAd() in onAdDismissedFullScreenContent():

@Override
public void onAdDismissedFullScreenContent() {
    // Called when ad is dismissed.
    // Set the ad reference to null so you don't show the ad a second time.
    Log.d(TAG, "Ad dismissed fullscreen content.");
    rewardedAd = null;
    MyApplication.getInstance().loadAwardAd();
}

I guess the reason is that I use new Myapplication().loadAwardAd(), but I must use the static method in showRewardedVideo() My showRewardedVideo() is:

public static void showRewardedVideo() {
    if (rewardedAd == null) {
        Log.d("TAG", "The rewarded ad wasn't ready yet.");
        return;
    }

    rewardedAd.setFullScreenContentCallback(new FullScreenContentCallback() {
        @Override
        public void onAdClicked() {
            // Called when a click is recorded for an ad.
            Log.d(TAG, "Ad was clicked.");
        }

        @Override
        public void onAdDismissedFullScreenContent() {
            // Called when ad is dismissed.
            // Set the ad reference to null so you don't show the ad a second time.
            Log.d(TAG, "Ad dismissed fullscreen content.");
            rewardedAd = null;
            MyApplication.getInstance().loadAwardAd();
        }

        @Override
        public void onAdFailedToShowFullScreenContent(AdError adError) {
            // Called when ad fails to show.
            Log.e(TAG, "Ad failed to show fullscreen content.");
            rewardedAd = null;
        }

        @Override
        public void onAdImpression() {
            // Called when an impression is recorded for an ad.
            Log.d(TAG, "Ad recorded an impression.");
        }

        @Override
        public void onAdShowedFullScreenContent() {
            // Called when ad is shown.
            Log.d(TAG, "Ad showed fullscreen content.");
        }
    });


    Activity activityContext = (AppActivity)MyApplication.getInstance().mainActive;
    // Make sure to operate on the UI thread
    activityContext.runOnUiThread(new Runnable() {
        @Override
        public void run() {
            rewardedAd.show(activityContext, new OnUserEarnedRewardListener() {
                @Override
                public void onUserEarnedReward(@NonNull RewardItem rewardItem) {
                    // Handle the reward.
                    Log.d(TAG, "The user earned the reward.");
                    int rewardAmount = rewardItem.getAmount();
                    String rewardType = rewardItem.getType();
                }
            });
        }
    });
}

This code refer to the official example code: GitHub. The only different place is I use the static method to show the awarded ad. But the cocos' creator only can use the static method. I don't know how to solve it. I