Problems with LoadLevelAsync and Android pro.

I have a problem when the program load scenes. Sometimes works and sometimes not. I have this sequence

Scene1 ---> Scene2 --async--> Scene3 ---> Scene1 ---> Scene2 --async--> Scene3

Sometimes the first LoadLevelAsync works fine and the second not. Scene3 is the game, and it has an scenario, characters, a lot of stuff. When I back from that scene to scene1 and then on scene2 call LoadLevelAsync, always freezes. I dont know why, because, previously the first asyncload worked fine.

The code is this:

 using UnityEngine;
 using System.Collections;
 
 public class LevelLoader : MonoBehaviour {
 
     static public string LevelToLoad="Menu";
     public AsyncOperation async = null; 
     private float progreso = 0.0f;
     private bool ejecutado = false;
 
     IEnumerator Start() {
         async = Application.LoadLevelAsync(LevelToLoad);
         async.allowSceneActivation = false;
         yield return async;
     }
 
     void FixedUpdate () {
         if (async != null){
             progreso = async.progress;
             transform.localScale = new Vector3(progreso, 1, 1);
             if (progreso>=0.9f) {
                 transform.localScale = new Vector3(1, 1, 1);
                 if(!ejecutado){
                     ejecutado = true;
                     Invoke("LoadLevel",1f);
                 }
             }
         }
     }
 
     void LoadLevel(){
         async.allowSceneActivation = true;
     }
 
 }
 

And the call to execute this script is this:

         LevelLoader.LevelToLoad = "Menu";
         Application.LoadLevel("LevelLoader");

Does anybody had problems with LoadLevelAsync on Android?

Regards and sorry, I have a better spanish :P