admob problem

When a ball is destroyed, an admob ad is displayed. I want the admob ad to be seen after all the balls are gone

using System.Collections; using System.Collections.Generic; using UnityEngine; using UnityEngine.SceneManagement;

public class gameover : MonoBehaviour {

 private List<GameObject> collidedObjects = new List<GameObject>();

 private void OnCollisionEnter(Collision collision)
 {
     
     



     if (collision.gameObject.CompareTag("g") && !collidedObjects.Contains(collision.gameObject))
     {

         collidedObjects.Add(collision.gameObject);

         if (collidedObjects.Count == 3)
             
         {

             SceneManager.LoadScene("GameOverLoad");
             
             

         }
         
         AdManager.instance.ShowInterstitial();

     }

     
 }
 private void Start()
 {
     AdManager.instance.RequestInterstitial();
 }

and AdManager.cs

using UnityEngine; using GoogleMobileAds.Api; using System;

public class AdManager : MonoBehaviour { private BannerView bannerAd; static bool bannerAdRequested = false;

 private InterstitialAd interstitial;



 public static AdManager instance;
 private void Awake()
 {
     if (instance == null)
     {
         instance = this;
     }
     else
     {
         Destroy(gameObject);
         return;
     }
 }
 void Start()
 {
     MobileAds.Initialize(InitializationStatus => { });
     // Get singleton reward based video ad reference.


     // RewardBasedVideoAd is a singleton, so handlers should only be registered once.



     if (bannerAdRequested)
         return;

     this.RequestBanner();
     bannerAdRequested = true;
 }

 private AdRequest CreateAdRequest()
 {
     return new AdRequest.Builder().Build();
 }
 private void RequestBanner()
 {
     string adUnitId = "my id";
     this.bannerAd = new BannerView(adUnitId, AdSize.SmartBanner, AdPosition.Bottom);

     // Clean up banner ad before creating a new one.
     if (this.bannerAd != null)
     {
         this.bannerAd.Destroy();
     }

     // Create a 320x50 banner at the top of the screen.
     this.bannerAd = new BannerView(adUnitId, AdSize.SmartBanner, AdPosition.Bottom);

     // Load a banner ad.
     this.bannerAd.LoadAd(this.CreateAdRequest());
 }
 public void RequestInterstitial()
 {

     string adUnitId = "my id";
     // Clean up interstitial ad before creating a new one.
     if (this.interstitial != null)
         this.interstitial.Destroy();

     // Create an interstitial.
     this.interstitial = new InterstitialAd(adUnitId);

     // Load an interstitial ad.
     this.interstitial.LoadAd(this.CreateAdRequest());
 }

 public void ShowInterstitial()
 {
     if (this.interstitial.IsLoaded())
     {
         interstitial.Show();
     }
     else
     {
         Debug.Log("Inerstitial Ad is not ready yet");
     }
 }

}