Android: AdMob not working

Just adding this in case someone else has the same problem as mine.

I tried using a new Ads Unit ID and that didn't work as well. In the end, the issue was because I was trying to fix an app that got suspended from Google Play and Admob had also actually stopped serving ads to applications with the specific package name / bundle ID.

Changing the package name of the application solved the issue for me.

I don't find any issue with your code/layout. Issue could be with the Ads Unit Id.

I would suggest you to try creating a different Ads Unit Id

Hope it works.

Be careful of the ad-unit id string. My error was a space in the end of the adUnitId

In your layout file (activity_welcome.xml), the adview container (RelativeLayout) layout width/height looks strange to me. And I think it is useless in your use case.
Change from:

<RelativeLayout
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:layout_alignParentBottom="true" >

    <com.google.android.gms.ads.AdView
        android:id="@+id/welcomeAdView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        ads:adSize="BANNER"
        ads:adUnitId="pub-***************" />
</RelativeLayout>

to:

<com.google.android.gms.ads.AdView
    android:id="@+id/welcomeAdView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    ads:adSize="BANNER"
    ads:adUnitId="pub-***************" />