Unity test ads loading on click but real ads taking minutes to load

I am creating a coins based app, where a user click on activity and then ads comes up instantly so that user can continue the app, but this think is working best in unity test ads but when test mode is off, unity interstitial ads are taking 3-4 minutes to load and my users are getting coins for free, so how to fix that thing?

my ads code :-

 if (UnityAds.isReady (interstitial)) {
        Log.d(TAG, "showAd: ad ready and showing...")
        UnityAds.show (activity, interstitial)
    }