Mopub: Facebook native ads no impression

Question

I integrated mopub native ads successfully on Android project. But Facebook native ads impressions are not counted/triggered. Impressions are not triggered in following event listener.

nativeAd.prepare(adView);
nativeAd.renderAdView(adView);
nativeAd.setMoPubNativeEventListener(new NativeAd.MoPubNativeEventListener() {
    @Override
    public void onImpression(View view) {
        Log.w(this.getClass().getSimpleName(), "impression");
    }

    @Override
    public void onClick(View view) {
        Log.w(this.getClass().getSimpleName(), "clicked");
    }
});

I tested on FacebookNative.java file. Following code is not triggered

@Override
        public void onLoggingImpression(final Ad ad) {
            notifyAdImpressed();
        }

Please guide, why fb native ads are not counting impressions.