Interstitial Ads not display

Now I'm using Unity 5.5.5f1 and GoogleMobileAds 3.2.0 but when I call function showInterstitialAd() nothing happened. This is my source code.

 using UnityEngine;
 using System.Collections;
 using GoogleMobileAds.Api;
 public class ShowAdsManager : MonoBehaviour {
     public string idAdMob = "";
     public static ShowAdsManager instance = null;
     InterstitialAd interstitial;
 
     void Awake()
     {
         if (instance == null) {
             instance = this;
         }
         else if (instance != this)
             Destroy(gameObject);    
         DontDestroyOnLoad(gameObject);
     }
 
 
     // Use this for initialization
     void Start () {
         RequestInterstitialAds ();
     }
 
     private void RequestInterstitialAds()
     {
         interstitial = new InterstitialAd(idAdMob);
         AdRequest request = new AdRequest.Builder().Build();
         interstitial.OnAdClosed += Interstitial_OnAdClosed;
         interstitial.LoadAd(request);
     }
 
     private void Interstitial_OnAdClosed(object sender, System.EventArgs e)
     {
         this.RequestInterstitialAds ();
 
     }
 
     public void showInterstitialAd() {
         if (interstitial.IsLoaded()) {
             interstitial.Show();
         }
     }
 }