RevMob session start acknoledgement

I want to put RevMob ads into my game. For this I used following website help to move ahead.

RevMob Integration SDK

I have written complete script for it. I want to preload fullscreen ad then I want to show it. This reason I want to detect session is started and then I want to pass ad request.

Here is my full script :

 private static readonly Dictionary<String, String> REVMOB_APP_IDS = new Dictionary<String, String> () {
         { "Android", "copy your Android RevMob Media ID here"},
         { "IOS", "copy your Android RevMob Media ID here" }
     };
     private RevMob revmob;
     private RevMobFullscreen fullscreen;
 
     void Awake ()
     {
         instance = this;
         revmob = RevMob.Start (REVMOB_APP_IDS, "AppController");
     }
 
     private void RequestRevMobFullScreen ()
     {
         Debug.Log("request revmob full screen method called......");
         fullscreen = revmob.CreateFullscreen ();
     }
 
     public void ShowRevMobFullScreen ()
     {
         if (fullscreen != null)
             fullscreen.Show ();
         else
             Debug.Log("show revmob full screen method called......");
     }
 
     #region IRevMobListener implementation
     public void SessionIsStarted ()
     {
         Debug.Log (">>> Session Started");
         RequestRevMobFullScreen ();
         revmob.SetTestingMode(RevMob.Test.WITH_ADS);
     }
 
     public void UserClosedTheAd (string revMobAdType)
     {
         Debug.Log (">>> AdClosed: " + revMobAdType);
         RequestRevMobFullScreen ();
     }

In above code, SessionIsStarted() method is listener not called any time so my RequestRevMobFullScreen() method was not called.

Basically I just want to preload my ads. If you have any strategy then please suggest it to me.