Admob reward ad only show once for a session

RewardBasedVideoAd rewardBasedVideo;

	void Start () {
		rewardBasedVideo = RewardBasedVideoAd.Instance;

		rewardBasedVideo.OnAdRewarded += HandleRewardBasedVideoRewarded;
		rewardBasedVideo.OnAdLoaded += HandleRewardBasedVideoLoaded;		
		rewardBasedVideo.OnAdFailedToLoad += HandleRewardBasedVideoFailedToLoad;
		rewardBasedVideo.OnAdClosed += HandleRewardBasedVideoClosed;
        rewardBasedVideo.OnAdLeavingApplication += HandleRewardBasedVideoLeftApplication;
	public void HandleRewardBasedVideoRewarded(object sender, Reward args)
	public void HandleRewardBasedVideoLoaded(object sender, EventArgs args)
	public void HandleRewardBasedVideoFailedToLoad(object sender, AdFailedToLoadEventArgs args){}
	public void HandleRewardBasedVideoClosed(object sender, EventArgs args){}
	public void HandleRewardBasedVideoLeftApplication(object sender, EventArgs args){}
	public void Click(){			
		string adUnitId = "ca-app-pub-3940256099942544/5224354917";
		AdRequest request = new AdRequest.Builder().Build();
		rewardBasedVideo.LoadAd(request, adUnitId);

This is my code snippet. Click() function calls when the button is clicked. But the problem is reward ad only shows up once for a session.
The game should close completely and start again and click the button to see the reward ad again.
I just want to show up the reward ad many times as the user clicks the button. Can you please give me a solution for that?

Same problem here. Up