Game Center Authentication More than Once Swift

Question

My code is as follows:

func authenticateLocalPlayer() { 
    let localPlayer: GKLocalPlayer = GKLocalPlayer.localPlayer()
    localPlayer.authenticateHandler = {(ViewController, error) -> Void      in
        if((ViewController) != nil) {
            self.presentViewController(ViewController, animated: true, completion: nil)
        } else if (localPlayer.authenticated) {
            print("Local player already authenticated")
            self.gcEnabled = true
            if self.gcEnabled == true {
                self.showLeaderboard()
                self.activity.stopAnimating()
            }
            // Get the default leaderboard ID
            localPlayer.loadDefaultLeaderboardIdentifierWithCompletionHandler({ (leaderboardIdentifer: String!, error: NSError!) -> Void in
                if error != nil {
                    println(error)
                } else {
                    self.gcDefaultLeaderBoard = leaderboardIdentifer
                }
            })
        }

            else {
                self.gcEnabled = false
                print("Local player could not be authenticated, disabling game center")
                print(error)
                if((ViewController) != nil) {
                self.presentViewController(ViewController, animated: true, completion: nil)
            }

        } } }  

func showLeaderboard() {

    let gcVC: GKGameCenterViewController = GKGameCenterViewController()
    gcVC.gameCenterDelegate = self
    gcVC.viewState = GKGameCenterViewControllerState.Leaderboards
    gcVC.leaderboardIdentifier = leaderboardIdentifier
    self.presentViewController(gcVC, animated: true, completion: nil)
}

This code works perfectly fine the first time my leaderboard button is clicked (in which I use an @IBAction and just call this function), however, if I am prompted to log in and I click "cancel", and then click the button again, it does not work. (meaning the log in page does not appear). Any ideas? Thanks for your help!