AVSpeechUtterance and no speech / missing speech in Swift

I'm currently putting together my first app using XCode 9.4.1 and Swift. Within my code, I am working my way through a list and I would like each value in the list to be spoken aloud. Below is my code for this

// classificationResults is my list
for returnedValue in self.classificationResults{
                    let utterance = AVSpeechUtterance(string: returnedValue)
                    utterance.voice = AVSpeechSynthesisVoice(language: "en-GB")
                    utterance.rate = 0.5

                    let synthesizer = AVSpeechSynthesizer()
                    synthesizer.speak(utterance)
                    // print(returnedValue)

When running this in the simulator, it works however it only says allowed the first one or two values out of the list (there are 7). When I then run this directly on my device, nothing is said aloud.

Any ideas / suggestions? Thanks in advance

I have implemented following code to make my all text to voice conversion by using AVSpeechSynthesizer. This code as a example, you can try by your code.

class TextToVoiceVC: UIViewController, AVSpeechSynthesizerDelegate {

    var arrSpeechCount = ["One", "Two", "Three", "Four", "Five", "Six", "Seven"]
    var count : Int = 0
    let speechSynthesizer   = AVSpeechSynthesizer()        

    //----------------------------------------------------------------
    // MARK:- AVSpeechSynthesixerDelegate
    //----------------------------------------------------------------

    func speechSynthesizer(_ synthesizer: AVSpeechSynthesizer, didStart utterance: AVSpeechUtterance) {

    }

    //----------------------------------------------------------------

    func speechSynthesizer(_ synthesizer: AVSpeechSynthesizer, didFinish utterance: AVSpeechUtterance) {

        speechSynthesizer.stopSpeaking(at: .word)

        count += 1
        if count < arrSpeechCount.count {
            let speechUtterance = AVSpeechUtterance(string: (arrSpeechCount[count]))
            DispatchQueue.main.async {
                self.speechSynthesizer.speak(speechUtterance)
            }
        }
    }


    //----------------------------------------------------------------
    // MARK:- View Life Cycle Methods
    //----------------------------------------------------------------

    override func viewDidLoad() {
        super.viewDidLoad()
        speechSynthesizer.delegate = self
    }

    //----------------------------------------------------------------

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

        // Code to start speech
        if speechSynthesizer.isSpeaking {
            speechSynthesizer.stopSpeaking(at: .immediate)
        } else {
            let speechUtterance = AVSpeechUtterance(string: (arrSpeechCount[count]))

            DispatchQueue.main.async {
                self.speechSynthesizer.speak(speechUtterance)
            }
        }
    }
}

This code is working in my both simulator as well as device. I hope this will useful to you also. You should try delegate with your data.