Unity 5 GUI Buttons Won't Show When Built

When building the game, the buttons (start, controls, quit) won't show up - they only will if I test it inside Unity.

I've tried re positioning in all sorts of ways (scale, location, etc.), changing the canvas to Screen Overlay - Camera... Nothing works. I have the Title (just a static image) and that shows up plus the background image.

This is for Windows and is 2D, by the way.

Here's the code:

 `using UnityEngine;
 using UnityEngine.UI;// we need this namespace in order to access UI elements within our script
 using System.Collections;
 using UnityEngine.SceneManagement;
 
 public class mainMenu : MonoBehaviour 
 {
     public Button startText;
     public Button exitText;
     public Button controlsText;
     public Button exitControlsText;

 void Start ()

 {
     startText = startText.GetComponent<Button> ();
     exitText = exitText.GetComponent<Button> ();

 }
     

 public void StartLevel () //this function will be used on our Play button to load the first level

 {
     SceneManager.LoadScene(2); //this will load our first level, which is "1"

 }

 public void Controls () //this function will be used on our Play button

 {
     SceneManager.LoadScene (1); //this will load the Controls scene from our build settings, which is "1"

 }

 public void ExitGame () //This function will be used on our "Quit" button in the main menu

 {
     Application.Quit(); //this will quit our game. Note this will only work after building the game

 }

 public void ExitControls () //this is the exit controls button

 {
     SceneManager.LoadScene (0); //this will load the main menu
 }

}