How to Show Admob interstitial ads more one time after Gameover?

using UnityEngine; using System.Collections; using GoogleMobileAds.Api;

public class AdManager : MonoBehaviour {

 public static AdManager Instance{set;get;}
 public string InterstitialId;
 public string BannerId;

 private BannerView bannerView;
 private InterstitialAd interstitial;

 public int gameoverToShowAd;
 public static int gameoverCounter;

 // Use this for initialization
 private void Start () {

     Instance = this;
     DontDestroyOnLoad (gameObject);

     bannerView = new BannerView (BannerId, AdSize.Banner, AdPosition.Bottom);
     interstitial = new InterstitialAd (InterstitialId);

     ShowBanner();

     LoadVideo();
 }

 private void Update()
 {
     if(gameoverCounter == gameoverToShowAd)
     {
         ShowVideo();
         gameoverCounter = 0;

     }
 }

 public void LoadVideo() {
     AdRequest request = new AdRequest.Builder ().Build ();
     interstitial.LoadAd (request);

 }

 public void ShowVideo()
 {
     if (interstitial.IsLoaded())
         interstitial.Show();
     else
     {
         AdRequest request = new AdRequest.Builder().Build();
         interstitial.LoadAd(request);
     }
 }

 public void ShowBanner()
 {
     AdRequest request = new AdRequest.Builder ().Build ();
     bannerView.LoadAd (request);

     bannerView.Show ();
 }

 public void RemoveBanner()
 {
     bannerView.Hide ();
 }

}

I just set 3 times Gameover to show admob interstitial ads, now just show one time, not every 3 gameover to show ads, please help, thanks!