UWP NativeAd returns NoAdAvailable on calling RequestAd for real values

Question

I am using Native Ads for my UWP app. The app works well with the test ad ids. However, when I put in real values for publishing (note the problem I am describing is for published app, and I downloaded the app from the Windows Store), it always returns NoAdAvailable error:

    private void LoadNativeAd(string adUnit, FrameworkElement adContainer)
    {
        var adsManager = new NativeAdsManagerV2(NativeAdAppId, adUnit);
        adsManager.ErrorOccurred += this.AdsManager_ErrorOccurred;

        adsManager.AdReady += (sender, e) =>
        {
            this.AdsManager_AdReady(adContainer, e);
        };

        adsManager.RequestAd();
    }

    private async void AdsManager_ErrorOccurred(object sender, NativeAdErrorEventArgs e)
    {
        var appFolder = ApplicationData.Current.LocalFolder;
        var file = await appFolder.CreateFileAsync("ad.log", CreationCollisionOption.OpenIfExists);

        using (var logStream = await file.OpenAsync(FileAccessMode.ReadWrite))
        {
            using (var streamWriter = new StreamWriter(logStream.AsStreamForWrite()))
            {
                streamWriter.WriteLine($"{e.ErrorCode}: {e.ErrorMessage} ({e.RequestId})");
            }
        }
    }

For testing value, AdsManager_AdReady works well but with real value (either on local testing or real published app) it is never called. The content of the ad.log file looks like this:

NoAdAvailable: No Ad Available (2ce6c6c3f848475f8dea0cc3cf2dba97)

The app/ad code is published for many weeks already. What's wrong with it?