IEnumerator skips remaining function after yield

Hi, excuse me for my english. I try to create a webgl with many Materials in it, so i need to download all the textures from the server. Now I have a problem with IEnumerator, as soon the first yield is in the Line of script, IEnumerator skips the whole remaining function. (the least function is, how i fixed it, but i know that's bad Fix) Can someone Help me please to Fix it?

     bool MaterialCreated = false;
     Material[] Material;
 
     //Material
     public Material[] getMaterial()
     {
         if(!MaterialCreated)
         {
             MaterialCreated = true;
             Material = new Material[1];
             Material[0] = Instantiate(Resources.Load("Material/Standard") as Material);
             StartCoroutine_Auto(downloadTex(TexturLink));                       
         }
         return Material;
     }
 
     //CreateMaterial if chosen
     private IEnumerator downloadTex(string urlstring)
     {
         WWW www = new WWW(urlstring);

         yield return www;
         if (string.IsNullOrEmpty(www.error))
         {
             Debug.Log("FAIL: " + urlstring);
         }
         else
         {
         Material[0].mainTexture = new Texture2D(1024, 1024, TextureFormat.RGB24, false, true);
         www.LoadImageIntoTexture((Texture2D)Material[0].mainTexture);
         Debug.Log("OK");
         }            
     }   


     //   It works like this, so I know that URL is working so far
     private IEnumerator downloadTex(string urlstring)
     {
         WWW www = new WWW(urlstring);
         while (www.progress < 1)
             Debug.Log(www.progress);
         Material[0].mainTexture = new Texture2D(1024, 1024, TextureFormat.RGB24, false, true);
         www.LoadImageIntoTexture((Texture2D)Material[0].mainTexture);
         Debug.Log("OK");
         
         yield return www;
     }