My admob interstitial ads only shows once

I have ad script attached to the scene where I want to display the ad (game over part) so when I display the ad it only shows once, when i play the game again the ad doesn't appear at the game over part.

     void OnEnabled () {
         RequestInterstitial ();
         ShowInterstitial ();
     
     }
 void Update () {
         if (shown == false) {
             ShowInterstitial();
             Debug.Log ("Not yet...");
         } 
 
     }
 
     private void RequestInterstitial()
     {
         //interstitial.Destroy ();
         //#if UNITY_ANDROID
         string adUnitId = "ca-app-pub-xxxxxxxxxxxxxxx/xxxxxxxxxx";
         //#elif UNITY_IPHONE
         //string adUnitId = "INSERT_IOS_INTERSTITIAL_AD_UNIT_ID_HERE";
         //#else
         //string adUnitId = "unexpected_platform";
         //#endif
 
         // Initialize an InterstitialAd.
          interstitial = new InterstitialAd(adUnitId);
         // Create an empty ad request.
 
     
         AdRequest request = new AdRequest.Builder().Build();
         // Load the interstitial with the request.
         interstitial.LoadAd(request);
 
     
 
     }
     private void ShowInterstitial() { 
         
         if (interstitial.IsLoaded()) { 
             interstitial.Show();
             interstitial.Destroy ();
             shown = true;
 
         }
         else    {
             Debug.Log ("Interstitial is not ready yet.");
         }
     }
 }

Nothing fancy I'm using the simple code for interstitial ads