AssetBundle dependency issue

If i load AssetBundle view.ab only,Resources.UnloadUnusedAssets will work fine. but when i load AssetBundle view.ab has dependencies with font.ab,Resources.UnloadUnusedAssets does not work any more. This is the code:

 WWW fontwww = new WWW("file:///" + Application.streamingAssetsPath + "/font.ab");
         yield return fontwww;
         var font = fontwww.assetBundle;
         WWW www = new WWW("file:///" + Application.streamingAssetsPath + "/view.ab");
         yield return www;
         var assetBundle = www.assetBundle;
         var request = assetBundle.LoadAllAssetsAsync();
         yield return request;
         Debug.Log("Load over");
         yield return new WaitForSeconds(3.0f);
         GameObject obj = GameObject.Instantiate(request.allAssets[0]) as GameObject;
         assetBundle.Unload(false);
 
         yield return new WaitForSeconds(3.0f);
         Debug.Log("Clear");
 
         GameObject.Destroy(obj);
         yield return new WaitForSeconds(1.0f);
         Resources.UnloadUnusedAssets();