Not download/load from cache for second time

Hi all, Here i created a assetbundle for scene by scene. I its perfectly loaded the scene first time but it doesn't load or even download the assetbundle,

 using UnityEngine;
 using System.Collections;
 
 public class Orbitrek_Elite : MonoBehaviour 
 {
     private string progress = " "; 
     private string logtxt;
     private string url = ;
     
     GUIStyle Font_Size;
 
     void Awake()
     {
         Font_Size = new GUIStyle ();
         Font_Size.fontSize = 40;
         Font_Size.normal.textColor = Color.black;
     }
 
     void log( string t )
     {
         
         logtxt += t + "/n";
     }
     void OnGUI()
     {
         Vector3 scale;
         
         float ResolutionX   = 1024;
         float ResolutionY   = 768;
         
         scale.x             = (float)Screen.width/ResolutionX;
         scale.y             = (float)Screen.height/ResolutionY;
         scale.z             = 1;
         
         Matrix4x4 svMat     = GUI.matrix;
         GUI.matrix          = Matrix4x4.TRS(new Vector3(0,0,0),Quaternion.identity,scale);
         
         GUI.Label (new Rect (560, 684, Screen.width - 5, Screen.height - 5), "Downloading: ", Font_Size);
         GUI.Label (new Rect(795, 684, Screen.width - 5, Screen.height - 5), logtxt + " " + progress, Font_Size);
     }
     
     IEnumerator Start()
     {
         using(WWW www = WWW.LoadFromCacheOrDownload (url, 1))
         { 
             while( !www.isDone )
             {
                 progress = " " + (www.progress * 100.0f).ToString("#") + " %";
                 yield return null;
             }
             yield return www;
             AssetBundle bundle = www.assetBundle;
             bundle.LoadAll();
             Application.LoadLevel("Orbit");
         }
     }
 }

In this script i try to load the scene, but its working smoothly when i load all scene in one time.