Admob Banner Ads not shown but are clickable

Occasionally the ad will appear very briefly when the screen is redrawn, but the rest of the time the ad is invisible. However, it is clickable and registering impressions. Tested on a Samsung Galaxy S5 and android-studio emulator. I am using Unity Editor 5.6 on Linux.

Here is my ad code, attached to the main camera:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using GoogleMobileAds;
 using GoogleMobileAds.Api;
 
 //SEE https://github.com/unity-plugins/Unity-Admob
 
 public class AdmobController : MonoBehaviour {
     private BannerView banner;
     // Use this for initialization
     void Start () {
     RequestBanner();
     ShowBanner();
     }
     
     void RequestBanner(){
         string bannerId = "ca-app-pub-xxxxxxxxxxxxx/xxxxxxxxxx";
     banner = new BannerView(bannerId,AdSize.Banner,AdPosition.Bottom);
     AdRequest adRequest = new AdRequest.Builder().Build();
     banner.LoadAd(adRequest);
     }
 
     public void ShowBanner(){
         banner.Show();
     }
 
 }

EDIT:

I tested the "Hello" sample app from https://github.com/googleads/googleads-mobile-unity, exact same issue. Interstitial ads show, banner ads do not show but are clickable.