Why my iAd banner not showing on my gameView?

I just finished my first game and I'm trying this code on my viewController to integrate an ad banner :

import iAd

class GameViewController: UIViewController {

override func viewDidLoad() {
    super.viewDidLoad()

    var skView = self.originalContentView as SKView
    var scene:SKScene = GameScene(size: skView.bounds.size)

    skView.showsFPS = false
    skView.showsNodeCount = false
    skView.showsPhysics = false


    let turbulence = SKFieldNode.turbulenceFieldWithSmoothness(0.1, animationSpeed: 0.1)
    scene.addChild(turbulence)
    let noise = SKFieldNode.turbulenceFieldWithSmoothness(0.4, animationSpeed: 0.1)
    scene.addChild(noise)

    skView.presentScene(scene)

    self.canDisplayBannerAds = true
}

On my firstViewController (the game presentation) it works but not on my gameViewController and I can't understand why.

Someone can help me ?

Thank you all !!!