Ionic 4 won't show ads but in console is successful

I am trying to get Admob in my app, I get console log success message but no ad showing in my app.

This is what I have:

app.module

import { AdMobFree } from '@ionic-native/admob-free/ngx';

providers: [
    AdMobFree,
]

posts.page.ts

import { AdMobFree, AdMobFreeBannerConfig } from '@ionic-native/admob-free/ngx';

constructor(
    private admobFree: AdMobFree
) { }

ionViewDidEnter() {
    this.showBannerAds();
}

showBannerAds() {
    const bannerConfig: AdMobFreeBannerConfig = {
      id: 'ca-app-pub-xxxxxx/xxxxxxxxxx',
      isTesting: false,
      autoShow: true,
      bannerAtTop: false
    };
    this.admobFree.banner.config(bannerConfig);
    console.log('start to prepare banner ads...'); //shows in console

    this.admobFree.banner.prepare()
      .then(() => {
        console.log('start to show banner ads...'); //shows in console
        this.admobFree.banner.show();
      })
      .catch(err => {
        console.log('show banner ads error: ', err);
      });
}

PS:
At the first I thought maybe i need to add my app in google play then they show, but even that my app is live on google play store yet no ads showing.

Any idea what could be wrong?

Anyone??