Admob interstitial ad show one time and not showing again

This is the code admob interstitial ad show only once what is wrong in this script

using UnityEngine;
using System.Collections;
using GoogleMobileAds.Api;

public class AdmobScript : MonoBehaviour
{
	InterstitialAd interstitial;

    // Use this for initialization
    void Start()
    {
        //Request Ads
        RequestBanner();
		RequestInterstitial();
    }

    public void showInterstitialAd()
    {
        //Show Ad
        if (interstitial.IsLoaded())
        {
            interstitial.Show();
        }

    }

   private void RequestBanner()
	{
		#if UNITY_EDITOR
			string adUnitId = "unused";
		#elif UNITY_ANDROID
			string adUnitId = "ca-app-pub-4697579006360442/5518347117";
		#elif UNITY_IPHONE
			string adUnitId = "INSERT_IOS_BANNER_AD_UNIT_ID_HERE";
		#else
			string adUnitId = "unexpected_platform";
		#endif

		// Create a 320x50 banner at the bottom of the screen.
		BannerView bannerView = new BannerView(adUnitId, AdSize.Banner, AdPosition.Bottom);
		// Create an empty ad request.
		AdRequest request = new AdRequest.Builder().Build();
		// Load the banner with the request.
		bannerView.LoadAd(request);
	}
	
	private void RequestInterstitial()
	{
		#if UNITY_ANDROID
			string adUnitId = "ca-app-pub-4697579006360442/1613571751";
		#elif UNITY_IPHONE
			string adUnitId = "INSERT_IOS_INTERSTITIAL_AD_UNIT_ID_HERE";
		#else
			string adUnitId = "unexpected_platform";
		#endif

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

}

showing ad and request ad again.

void ad ()
{
Admanager.instance.ShowFullScreenAd();
Admanager.instance.RequestFullScreenAd();
}