Unity3d Intersititial ads not showing evertime

Hello, I have interesting problem.

	if (adCount % 3 == 0) {
		if (interstitial.IsLoaded ()) {
			interstitial.Show ();
			Debug.Log ("interstitial must be shown");
		} else {
			Debug.Log ("!interstitial.IsLoaded ()");
		}
	}

My ads are not showing. When I log in terminal, I see “!interstitial.IsLoaded ()”.

I see my ad 1 in about 25 times, it will be 1 in 3 times.

Also here is my stats(daily). It gives a opinion to you.
https://s12.postimg.org/8aluplbkt/Screen_Shot_2017_02_13_at_13_35_09.png

What is the problem? Why I can’t see my interstital ads?

Admob.getInstance().loadInterstitial ();//load ad

void onInterstitialEvent(string eventName, string msg)
    {
        Debug.Log("handler onAdmobEvent---" + eventName + "   " + msg);
        if (eventName == AdmobEvent.onAdLoaded)
        {
            Admob.Instance().showInterstitial();
        }
}

you have not call loadInterstitial

download the souce code