assigning MovieTexture

I'm trying to dynamically load a video from StreamingAssets folder, but somehow the www.movie returns a movie texture with duration -1

Code (csharp):

1.  
2.     public    MovieTexture     movTex;
3.     void Start ()
4.     {
5. #if UNITY_EDITOR
6.         LoadIntroMovie();
7. #else
8.        // Play intro movie on Handheld device
9. #endif
10.     }
11.  
12. void LoadIntroMovie()
13.     {
14.         movTex = null;
15.  
16.         string videoFilePath = Application.streamingAssetsPath+"/Intro Videos/sample_mpeg4.mp4";
17.         if (! System.IO.File.Exists(videoFilePath) )
18.         {
19.               Debug.LogError(videoFilePath + " does not exist!");
20.               return;
21.         }
22.      
23.         WWW www = new WWW( "file://" + videoFilePath );
24.      
25.         if (www != null)
26.         {
27.             while ( !www.isDone ) Debug.Log("Loading data " + www.url + "...");
28.             Debug.Log(www.url + " loaded!");
29.             movTex = www.movie;
30.         }
31.  
32.         print("Movie texture duration : " + movTex.duration); // ERROR : Movie texture duration : -1
33.  

Any idea what went wrong?
- The video file is definitely not corrupted. It's the sample .mp4 file I downloaded from Quick Time website and it plays OK if I simply drag and drop the file into the quad I use as a movie plane
- The www loading method definitely work if I am about to fetch a text file (instead of movie)

 

bump.

Could anyone help me?