Interstitial Ads not showing

Could anyone please help me with my admob interstitial ads that arent working? I think its simple but for some reason its not working. Thank you in advance.

using UnityEngine;
using System.Collections;
using System.Collections.Generic;
using admob;


public class displayinterad : MonoBehaviour
{
	void Start ()
	{
		Admob.Instance().loadInterstitial(); 
		if (Admob.Instance ().isInterstitialReady ()) {
			Admob.Instance ().showInterstitial ();
		}
	}
}

You need to wait for the interstitial ad to be loaded before you check it is ready and show it.

Try something like this:

     void Start ()
     {
         Admob.Instance().loadInterstitial(); 
     }

     void Update()
     {
          if (Admob.Instance ().isInterstitialReady ()) {
             Admob.Instance ().showInterstitial ();
          }
      }

Ideally you should call loadInterstitial() earlier in your app (maybe when you initially load), and then check and show it later (maybe between levels?)