unity 5 2d game admob interstitial not working

my admob interstial not working on my game here is my code: can you tell me someone how to show interstitial on menu scene and when is game over to show immediately.

 private BannerView BannerView;
 private InterstitialAd interstitial;
 public menu bc;
 public Button[] buttons;
 public Text scoreText;
 bool gameOver;
 int score;

 // Use this for initialization
 void Start () {

     RequestBanner ();
     RequestInterstitial ();
     gameOver = false;
     score = 0;
     InvokeRepeating ("scoreUpdate", 1.0f, 0.5f);
 }


 // Update is called once per frame
 void Update () {
     scoreText.text = "Score: " + score;
 }


 void scoreUpdate(){
     if (gameOver == false) {
         score += 14;
     }
 }


 public void gameOverActivated(){
     gameOver = true;
     foreach (Button button in buttons) {
         button.gameObject.SetActive (true);
     }


         
 }
     


 public void play (){
     Application.LoadLevel ("level1");
     bc.bacSound.Stop ();
 }

 public void Pause (){
     if (Time.timeScale == 1) {
         Time.timeScale = 0;

         foreach (Button button in buttons) {
             button.gameObject.SetActive (true);

         }

ShowInterstitial (); } else if (Time.timeScale == 0) { Time.timeScale = 1; foreach (Button button in buttons) { button.gameObject.SetActive (false); } }

 }


 public void Replay(){
     Application.LoadLevel (Application.loadedLevel);
     if (Time.timeScale == 0) {
         Time.timeScale = 1;
     }

 
 
 }


 public void Menu(){
     Application.LoadLevel ("menuScene");
     ShowInterstitial ();
     if (Time.timeScale == 0) {
         Time.timeScale = 1;
     }
 }


 public void Exit(){
     Application.Quit ();

 }

 private void RequestBanner()
 {
     #if UNITY_EDITOR
     string adUnitId = "unused";
     #elif UNITY_ANDROID
     string adUnitId = "ca-app-pub-3940256099942544/6300978111";
     #elif UNITY_IPHONE
     string adUnitId = "INSERT_IOS_BANNER_AD_UNIT_ID_HERE";
     #else
     string adUnitId = "unexpected_platform";
     #endif

     // Create a 320x50 banner at the top of the screen.
     BannerView bannerView = new BannerView(adUnitId, AdSize.Banner, AdPosition.Top);
     // Create an empty ad request.
     AdRequest request = new AdRequest.Builder().Build();
     bannerView.LoadAd(request);
     }

     private void RequestInterstitial()
     {
     #if UNITY_ANDROID
     string adUnitId = "ca-app-pub-3940256099942544/1033173712";
     #elif UNITY_IPHONE
     string adUnitId = "INSERT_IOS_INTERSTITIAL_AD_UNIT_ID_HERE";
     #else
     string adUnitId = "unexpected_platform";
     #endif

     // Initialize an InterstitialAd.
     InterstitialAd interstitial = new InterstitialAd(adUnitId);
     // Create an empty ad request.
     AdRequest request = new AdRequest.Builder().Build();
     interstitial.LoadAd(request);
     }

     private void ShowInterstitial ()
     {
     if (interstitial.IsLoaded ()) {
     interstitial.Show ();
     } else {
     print ("Interstitial is not ready yet.");
     }
     }

}