Xcode Swift - continuing animation after returning to view

Title might be a bit ambiguous so i'll get straight to the point. Within my Xcode project I've implemented a simple animation in my initial ViewController, which functions fine, but after leaving the initial View, and moving to another scene, and then returning to the initial view, the animation fails to restart and I can't figure out why.

Here's the relevant code from my initial view

     override func viewDidAppear(_ animated: Bool) {
     super.viewDidAppear(animated)

        //cloud animation
        let oldCenter = cloud.center
        let newCenter = CGPoint(x: oldCenter.x - 800, y: oldCenter.y)
        UIView.animate(withDuration: 30.0, delay: 0.0, options:
            [.curveLinear, .repeat], animations: {
                self.cloud.center = newCenter
        }, completion: nil)
        //second cloud animation
        let oldCenter2 = cloud2.center
        let newCenter2 = CGPoint(x: oldCenter2.x + 800, y: oldCenter2.y)
        UIView.animate(withDuration: 15.0, delay: 0.0, options:
            [.curveLinear, .repeat], animations: {
                self.cloud2.center = newCenter2
        }, completion: nil)

    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
    }