admob interstitial alway returns false

问题:

I am fairly new to AdMob. I tried looking up problem online and here but I can't find a solution that works for me.

So basically I am trying to create this game using Swift and in the gameviewcontroller I create 3 functions that create the ad and then I want to call them in my StartMenu scene.

Here is my functions for ad interstitial in gameviewcontroller:

func getInterstitialAd(){
    interstitial = GADInterstitial(adUnitID: "AdMob Unit ID")
    let interstitialRequest = GADRequest()
    interstitial.load(interstitialRequest)
}

func showAd() {
    print(interstitial.isReady)
    if interstitial.isReady {
        interstitial.present(fromRootViewController: self)
    } else {
        interstitial = createAd()
        print("Ad wasn't ready")
    }
}

func createAd() -> GADInterstitial{
    let interstital = GADInterstitial(adUnitID: "AdMob Unit ID")
    interstitial.load(GADRequest())
    return interstital
}

and in my StartMenu Scene:

 var viewController = GameViewController()

I have this in my game over function and it basically gets called when the player dies:

viewController.getInterstitialAd()
viewController.showAd()

And every time the player dies it prints false and ad wasn't ready.