How to solve Admob Error failed to load ad 0

Question

I have implemented Admob in my app and working well with test ads. I am loading ads in feed list, and getting error " Failed to load Ad: 0". According to Stack Overflow answers, this error mostly related to newly created ad units, but it had been more than 15 days but getting the same error. While rewarded video ad unit is working fine. Admob account is also with approved status. Asked the same question in Google Admob Community, but no answer.

Don't mark it duplicate of this question as the accepted answer says to wait, but in my case, I am waiting for last 15 days.

Here is code snippet.

private void showNativeGoogleAd(UnifiedNativeAdViewHolder holder)
{
    AdLoader adLoader = new AdLoader.Builder(mContext, mContext.getString(R.string.admob_social_wall_ad_unit_id))
    //AdLoader adLoader = new AdLoader.Builder(mContext, "ca-app-pub-3940256099942544/1044960115")
            .forUnifiedNativeAd(new UnifiedNativeAd.OnUnifiedNativeAdLoadedListener() {
                @Override
                public void onUnifiedNativeAdLoaded(UnifiedNativeAd unifiedNativeAd) {
                    // Show the ad.
                    populateNativeAdView(unifiedNativeAd, holder.adView);
                    //holder.fl_adplaceholder.removeAllViews();
                    //holder.fl_adplaceholder.addView(holder.adView);
                }
            })
            .withAdListener(new AdListener() {
                @Override
                public void onAdFailedToLoad(int errorCode) {
                    // Handle the failure by logging, altering the UI, and so on.
                    Log.d("TAG", "onAdFailedToLoad: Google => "+errorCode);
                }
            })
            .withNativeAdOptions(new NativeAdOptions.Builder()
                    // Methods in the NativeAdOptions.Builder class can be
                    // used here to specify individual options settings.
                    .build())
            .build();

    adLoader.loadAd(new AdRequest.Builder().build());
}

private void populateNativeAdView(UnifiedNativeAd nativeAd,
                                  UnifiedNativeAdView adView) {
    // Some assets are guaranteed to be in every UnifiedNativeAd.
    ((TextView) adView.getHeadlineView()).setText(nativeAd.getHeadline());
    ((TextView) adView.getBodyView()).setText(nativeAd.getBody());
    ((Button) adView.getCallToActionView()).setText(nativeAd.getCallToAction());

    // These assets aren't guaranteed to be in every UnifiedNativeAd, so it's important to
    // check before trying to display them.
    com.google.android.gms.ads.formats.NativeAd.Image icon = nativeAd.getIcon();

    if (icon == null) {
        adView.getIconView().setVisibility(View.INVISIBLE);
    } else {
        ((ImageView) adView.getIconView()).setImageDrawable(icon.getDrawable());
        adView.getIconView().setVisibility(View.VISIBLE);
    }

    if (nativeAd.getPrice() == null) {
        adView.getPriceView().setVisibility(View.INVISIBLE);
    } else {
        adView.getPriceView().setVisibility(View.VISIBLE);
        ((TextView) adView.getPriceView()).setText(nativeAd.getPrice());
    }

    if (nativeAd.getStore() == null) {
        adView.getStoreView().setVisibility(View.INVISIBLE);
    } else {
        adView.getStoreView().setVisibility(View.VISIBLE);
        ((TextView) adView.getStoreView()).setText(nativeAd.getStore());
    }

    if (nativeAd.getStarRating() == null) {
        adView.getStarRatingView().setVisibility(View.INVISIBLE);
    } else {
        ((RatingBar) adView.getStarRatingView())
                .setRating(nativeAd.getStarRating().floatValue());
        adView.getStarRatingView().setVisibility(View.VISIBLE);
    }

    if (nativeAd.getAdvertiser() == null) {
        adView.getAdvertiserView().setVisibility(View.INVISIBLE);
    } else {
        ((TextView) adView.getAdvertiserView()).setText(nativeAd.getAdvertiser());
        adView.getAdvertiserView().setVisibility(View.VISIBLE);
    }

    // Assign native ad object to the native view.
    adView.setNativeAd(nativeAd);
}