Admob on iOS and Android. Interstitial ads are showing but events are not firing.

When I tested demo scene from the admob plugin for unity in empty project - the interstitial ads were showing and events were fired well. But when I am trying to integrate the plugin with my game - the interstitial ads are showing but events are not firing. I thought that it caused because of another plugin integrated with my game, but I tried also remove all the plugins and integrate only admob, and interstitial ads are showing but events still are not firing.

Here is my AdMobManager: using System; using UnityEngine; using System.Collections; using GoogleMobileAds.Api;

 public class AdMobManager : MonoBehaviour {
 
     public static AdMobManager Instance{ get; private set; }
 
     private InterstitialAd _interstitial;
     private Action _callback;
 #if UNITY_ANDROID
     string adUnitId = "MyAdUnitId";
 #elif UNITY_IPHONE
     string adUnitId = "MyAdUnitId";
 #else
     string adUnitId = "unexpected_platform";
 #endif
 
     void Awake(){
         if (Instance == null)
             Instance = this;
         else if (Instance != null)
             Destroy (gameObject);
 
         DontDestroyOnLoad (gameObject);
     }
 
     void Start(){
 
 
         // Initialize an InterstitialAd.
         //_interstitial = new InterstitialAd(adUnitId);
 
         //SignEventsToHandlers ();
     }
 
     //Events handlers
     void OnAdClosed (object sender, System.EventArgs e){
         Debug.Log (string.Format ("AdMob.AdClosed - OK. sender: {0}, eventArgs: {1}", sender, e));
         if (_callback != null) _callback();
         else Debug.Log("_callback is null");
     }
 
     void OnAdClosing (object sender, System.EventArgs e){
         Debug.Log (string.Format ("AdMob.AdClosing - OK. sender: {0}, eventArgs: {1}", sender, e));
     }
 
     //void OnAdFailedLoad (object sender, AdFailedToLoadEventArgs e){
     //    Debug.Log (string.Format ("AdMob.AdFailedLoad - OK. sender: {0}, eventArgs: {1}", sender, e));
     //}
 
     //void OnAdLeftApplication (object sender, System.EventArgs e){
     //    Debug.Log (string.Format ("AdMob.AdLeftApplication - OK. sender: {0}, eventArgs: {1}", sender, e));
     //}
 
     private void OnAdLoaded(object sender, System.EventArgs e){
         Debug.Log(string.Format("AdMob.AdLoaded - OK. sender: {0}, eventArgs: {1}", sender, e));
     }
 
     private void OnAdOpened(object sender, System.EventArgs e){
         Debug.Log(string.Format("AdMob.AdOpened - OK. sender: {0}, eventArgs: {1}", sender, e));
     }
 
     //Public interface
     public void ShowInterstital(Action callback){
         //if (IapManager.Instance.IsXlPackBought) return;
         _callback = callback;
         if (_interstitial.IsLoaded())
             _interstitial.Show();
     }
 
     //TODO: Every time to show interstital animation I need to request this ad from server, check if it was loaded successfully and then show it
     //TODO: Or mayby to see on events. Maybe there event something like this: OnInterstitial loaded;
 
     //Actions
     public void RequestInterstitial(){
         // Create an empty ad request.
 #if UNITY_IOS
         AdRequest request = new AdRequest.Builder ().AddTestDevice (GameTools.GetIOSAdMobID ()).Build ();
         // Load the interstitial with the request.
         _interstitial.LoadAd(request);
 #endif
 #if UNITY_ANDROID
         _interstitial = new InterstitialAd(adUnitId);
         _interstitial.AdClosed += OnAdClosed;
         _interstitial.AdClosing += OnAdClosing;
         _interstitial.AdLoaded += OnAdLoaded;
         _interstitial.AdOpened += OnAdOpened;
         AdRequest request = new AdRequest.Builder()
                 .AddTestDevice(AdRequest.TestDeviceSimulator)
                 .AddTestDevice("E6CDB888557BA9A0") // My device ID
                 .AddKeyword("game")
                 .SetGender(Gender.Male)
                 .SetBirthday(new DateTime(1985, 1, 1))
                 .TagForChildDirectedTreatment(false)
                 .AddExtra("color_bg", "9B30FF")
                 .Build();
         _interstitial.LoadAd(request);
         
 #endif
     }