Pause menu script not working in built game (but works fine in editor)?

My game has a pause menu script (attached to the main camera in every scene) that pauses/unpauses the game when you press Escape and opens the menu too. However, in the built version of the game, it still pauses/unpauses, but the menu doesn't show! How might I fix that? Here's the script (C#), in case it helps you:

 using UnityEngine;
 using System.Collections;
 
 public class PauseMenu : MonoBehaviour {
     public GUISkin guiSkin;
     public float nativeVerticalResolution = 1200.0f;
     private float nativeHorizontalResolution = 1900f;
     private bool isPaused = false;
     public string levelName;
     
     private PlayerStatus status;
     
     void Start() {
         status = GameObject.FindGameObjectWithTag("Player").GetComponent<PlayerStatus>();
     }
     
     void Update() {
         if (Input.GetKeyDown("escape")) {
             print("Paused");
             isPaused = !isPaused;
             Screen.showCursor = isPaused;
             Pause.TogglePaused ();
         }
     }
     
     public void TogglePause() {
         isPaused = !isPaused;
     }
     
     private bool isQuit = false;
      
     void OnGUI (){
         // Set up gui skin
         GUI.skin = guiSkin;
         GUI.matrix = Matrix4x4.TRS (new Vector3(0, 0, 0), Quaternion.identity,  new Vector3 ((float) Screen.width / nativeHorizontalResolution, (float) Screen.height / nativeVerticalResolution, 1f));
         
         if(isPaused) {
             if (!Screen.showCursor) Screen.showCursor = true;
             // RenderSettings.fogDensity = 1;
             GUILayout.BeginArea(new Rect (480, 200, 1000, 1000));{
                 GUILayout.Box ("Game Paused", "Title");
                 
                 if (GUILayout.Button ("Reload Level")) {
                     Application.LoadLevel(levelName);
                     Time.timeScale = 1.0f;
                     isPaused = false;
                     Pause.TogglePaused ();
                 }
             
                 if (GUILayout.Button ( "Open Status Menu")) {
                     status.OpenStatGUI();
                     isPaused = false;
                 }
         
                 CameraControls.invertX = GUILayout.Toggle(CameraControls.invertX, "Invert Camera X Movement", "button");
                 CameraControls.invertY = GUILayout.Toggle(CameraControls.invertY, "Invert Camera Y Movement", "button");
             
                 if (GUILayout.Button ("Return to Game")) {
                     Time.timeScale = 1.0f;
                     isPaused = false;
                     Pause.TogglePaused ();
                 }
             
                 if (GUILayout.Button ("Quit Game")) {
                     //Time.timeScale = 1.0f;
                     isPaused = false;
                     isQuit = true;
                     Pause.TogglePaused ();
                 }
                 
             }GUILayout.EndArea();
 
         } else if (isQuit) {
             GUILayout.BeginArea(new Rect (480, 200, 1000, 1000));
                 GUILayout.Box ("Are you sure?", "Title");
 
                 if (GUILayout.Button ("Quit")) {
                     Time.timeScale = 1.0f;
                     isQuit = false;
                     Pause.TogglePaused ();
                     Application.LoadLevel("mainMenu");
                 }
             
                 if (GUILayout.Button ("Cancel")) {
                     isPaused = true;
                     isQuit = false;
                 }
             GUILayout.EndArea();
         }
     }
 }

Interestingly enough, there's another script that pauses the game and opens a menu (the stats menu mentioned in this script) when you press "L", and that menu works perfectly in the build.

EDIT: I've figured out that this OnGUI is not getting called at all. What could be doing that? I tried making it public, but that didn't change anything either.