Why does my AdMob banner not appear on my Samsung TabA?

Hey folks,

Doing a very basic test for AdMob banners. The app is some very simple UGUI, along with a tap counter. On top of this, I'm trying to display an AdMob banner - I've downloaded the plugin and I'm using sample code from this page, as follows:

Code (CSharp):

1. using System.Collections;
2.     using System.Collections.Generic;
3.     using UnityEngine;
4.     using GoogleMobileAds.Api;
5.  
6.     public class AdmobTest : MonoBehaviour {
7.  
8.         // Use this for initialization
9.         void Start () {
10.             RequestBanner();
11.         }
12.    
13.         // Update is called once per frame
14.         void Update () {
15.        
16.         }
17.  
18.         public void RequestBanner()
19.         {
20.             Debug.Log("Banner Ad has been requested.");
21.  
22.         #if UNITY_EDITOR
23.             string adUnitId = "unused";
24.     #elif UNITY_ANDROID
25.                 string adUnitId = "**ADMOB ID REMOVED FOR POSTING TO UNITY FORUMS**";
26.     #elif UNITY_IPHONE
27.                 string adUnitId = "INSERT_IOS_BANNER_AD_UNIT_ID_HERE";
28.     #else
29.                 string adUnitId = "unexpected_platform";
30.     #endif
31.  
32.             // Create a 320x50 banner at the top of the screen.
33.             BannerView bannerView = new BannerView(adUnitId, AdSize.Banner, AdPosition.Top);
34.             // Create an empty ad request.
35.             AdRequest request = new AdRequest.Builder().Build();
36.             // Load the banner with the request.
37.             bannerView.LoadAd(request);
38.        
39.             // bannerView.Show();
40.         }
41.     }

At the moment, the app builds and loads on my tablet, the debug message (Debug.Log("Banner Ad has been requested.")) can be read in the Unity Console, but no banner is visible.

Can anybody advise me on my next step? My intention is to ultimately use this code to display a banner ad above the restart / config buttons on the gameover screen of my app.

Thank you in advance,

--Rev