Google Admob Ad not showing

I am trying to implement Google Ads into my flutter app. I closely followed the implementation guides but when I start the app even though it says the ad successfully loaded it does not show up. Sometimes it flashes up for a second but immediately disappears afterwords. Does anyone have an idea what could be the problem?

    loadAd() {     BannerAd banner = BannerAd(       adUnitId: adUnitId,       request: AdRequest(),       size: AdSize.banner,       listener: BannerAdListener(           // Called when an ad is successfully received.           onAdLoaded: (ad) {         debugPrint('$ad loaded.');         setState(() {           _isLoaded = true;         });       },           // Called when an ad request failed.           onAdFailedToLoad: (ad, err) {         debugPrint('BannerAd failed to load: $err');         // Dispose the ad here to free resources.         ad.dispose();       }, )     );     banner.load();     setState(() {       _bannerAd = banner;     });   } 

 Widget build(BuildContext context) {     return _isLoaded         ? Container()         : Container(           alignment: Alignment.center,           child: SizedBox(             height: AdSize.banner.height.toDouble(),             width: AdSize.banner.width.toDouble(),             child: AdWidget(               ad: _bannerAd!,             ),           ),         );   }