Scripts don't work when i try app

I am doing an application for android. When i start the game on Unity, or i try play on it all script and icons are working, but when i build my app and try it on my phone 2 botton don't work (the button for exit game and the button for return to main menu). Help me please

N.B the button for return to main menu is in 2 scenes, in one is working, in the other no.

That are the scripts Quit:

 using UnityEngine;
 using System.Collections;
 
 public class Quit : MonoBehaviour {
     
     // Use this for initialization
     void Start () {
         
     }
     
     // Update is called once per frame
     void Update () {
         
     }
     void OnMouseDown() {
         Application.Quit();
     }
 }

Main menù

 using UnityEngine;
 using System.Collections;
 
 public class MainMenu : MonoBehaviour {
 
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
     void OnMouseDown() {
         if (GameObject.Find("?")!=null){
             GameObject.Find("?").audio.Stop();}
         Application.LoadLevel(0);
 
     }
 }