I am using admob with unity. But the ads are not getting loaded. My script is bellow.. Am i missing something? Or i have to do something else?

     float time;
     private InterstitialAd interstitialAd;
 
     // Use this for initialization
     void Start () {
         interstitialAd = new InterstitialAd("xxxxxx");
         LoadAd();
         time = Time.time + 30;
     }
 
     void LoadAd()
     {
         interstitialAd.LoadAd(new AdRequest.Builder().Build());
     }
 
     private void Update()
     {
         if(time<Time.time)
         {
             time += 20;
             if (interstitialAd.IsLoaded()) {
                 ShowReward();
                 LoadAd();
             }
             else
             {
                 MonoBehaviour.print("Not Loaded");
             }
         }
     }
 
     public void ShowReward()
     {
         interstitialAd.Show();
     }