iOS 3D assetbundle fail

I successfully load 2D assetbundle from server on iOS, android, editor. Everything is going well when I load 3D model in android and Editor. But it fails when load 3D model for iOS. I have separated build asset for iOS and Android.

here is my method, and :

 IEnumerator downloadAsset(BaseModel baseModel){
         string url = baseModel.url_android;
         #if UNITY_IOS
         url = baseModel.url_ios;
         #endif
         WWW download = new WWW(url);
         progressBar.SetActive (true);
         progressItem.anchorMax = new Vector2 (0, 1f);
         while (download.progress < 1 && download.error == null) {
             yield return new WaitForSeconds(2f);
             progressItem.anchorMax = new Vector2 (download.progress,1f);
         }
         progressBar.SetActive (false);
         yield return download;
         AssetBundle assetBundle = download.assetBundle;
         if (assetBundle != null) {
             // Alternatively you can also load an asset by name (assetBundle.Load("my asset name"))
             m2DPrefab = assetBundle.LoadAsset(baseModel.title) as GameObject; //iOS force stop here
             Debug.Log("Prefab created");
             if (m2DPrefab != null) {
                 InstantiateModel (m2DPrefab, baseModel.title);
                 //StartCoroutine(SetBackground(baseModel.url_background));
                 model.SetActive (true);
             } else
                 Debug.Log("Couldn't load resource");
         } else {
             Debug.Log("Couldn't load resource");    
         }
     }

Note : I can load 3D asset bundle after I create model on the scene before I deploy it to iOS, then load asset bundle with that code. It works smoothly. But.. I want make it dynamic without instantiate it first. I don't know how to fix it.

Please help. Thanks