Admob Ads do not show on some devices

I have Admob Ads on some of my apps and they work fine. Recently I changed my phone to Samsung Note 9 and then I realized that my ads are not showing up on this device. I tried with my old phone (Samsung S8) and same ads show up successfully on this device. Now I am trying to figure out whats wrong. If my Note 9 does not show ads, then some other devices may also be in the same position.

I use the same Google account on both devices. There's just one major difference; S8 is bought from US and Note 9 is from Turkey. But that shouldn't be an issue because my Note 9 shows ads from other apps.

How can I dig into this to understand and solve the issue ? Is there any address that I can raise a ticket to Google to understand the issue ?

I am trying with banner ads and here are my code;

 mAdView = (AdView) findViewById(R.id.adView);
 AdRequest adRequest = new AdRequest.Builder()
         .addTestDevice("A727CB6064156F49C550AF77325CF4D1")
         .build();
 mAdView.loadAd(adRequest);


<com.google.android.gms.ads.AdView
    xmlns:ads="http://schemas.android.com/apk/res-auto"
    android:id="@+id/adView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_below="@+id/imageView3"
    android:layout_marginTop="5dp"
    ads:adSize="BANNER"
    ads:adUnitId="ca-app-pub-1777459395877022/7665755111" >

error that I see :

"There was a problem getting an ad response. ErrorCode: 0 Failed to load ad:0"

Solution

the problem appeared to be a generic problem on Admob. I wrote to Admob support and they fixed it eventually. but it took several weeks.