My SerializeField won't show on my GameState script.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.SceneManagement;

public class GameState : MonoBehaviour {

    private bool gameStarted = false;

    [SerializeField]
    private Text gameStateText;

    [SerializeField]
    private GameObject player; 

    [SerializeField]
    private BirdMovement birdMovement;

    [SerializeField]
    private FollowCamera followCamera;

    private float restartDelay = 3f;

    private float restartTimer;

    private PlayerMovement playerMovement;

    private PlayerHealth playerHealth;

    // Use this for initialization
    void Start () {
        Cursor.visible = false;

        playerMovement = player.GetComponent<PlayerMovement> ();
        playerHealth = player.GetComponent<PlayerHealth> ();

        // Prevent the player from moving at the start of the game
        playerMovement.enabled = false;

        // prevent the bird from moving at the start of the game
        birdMovement.enabled = false;

        // Prevent the follow camera from moving to its game position
        followCamera.enabled = false;

    }

    // Update is called once per frame
    void Update () {

        // If the game is not started and the player presses the space bar...
        if (gameStarted == false && Input.GetKeyUp  (KeyCode.Space)) {

            // ...start the game.
            StartGame();

        }

        // If the player is no longer alive...
        if (playerHealth.alive == false) {

            // ...end the game...
            EndGame ();

            // ...increment a timer to count up to restarting
            restartTimer = restartTimer + Time.deltaTime;

            // ... and if it reaches the restart delay...
            if (restartTimer >= restartDelay) {

                //... then reload the currently loaded scene.
                SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex);

            }

        }

    }

    private void StartGame() {

        // Set the game state
        gameStarted = true;

        // Remove the start text
        gameStateText.color = Color.clear;

        //Allow the player to move
        playerMovement.enabled = true;

        // Allow the bird to move
        birdMovement.enabled = true;    

        // Allow the camera to move
        followCamera.enabled = true;

    }

    private void EndGame() {

        // set the game state
        gameStarted = false;

        // Show the game over text
        gameStateText.color = Color.white;
        gameStateText.text = "Game Over!";

        // Remove the player from the game
        player.SetActive (false);
    }

}