NGUI positioning over NPC

Hi.

As the question says, I'm trying to position a NGUI object over a NPC.

This is the code that I'm using to do it:

 private GameObject CreateOrderBalloon()
     {
         GameObject balloon = GameObject.Instantiate( _orderPrefab ) as GameObject;
         balloon.transform.parent = NGUITools.FindCameraForLayer( LayerMask.NameToLayer( "GUI2D" ) ).transform.parent;
         balloon.GetComponent<GUIOrderBalloon>().SetOrder( _currentOrder );
 
         Vector3 gui2dPos = Camera.main.WorldToScreenPoint( transform.position );
         Debug.Log(transform.position);
 
         gui2dPos.x -= (Screen.width / 2.0f);
         gui2dPos.y -= (Screen.height / 2.0f);
         gui2dPos.z = 0;
 
         Debug.Log(gui2dPos);
 
         balloon.transform.position = Camera.main.ScreenToWorldPoint(gui2dPos);
         balloon.transform.localRotation = Quaternion.identity;
         balloon.transform.localScale = Vector3.one;
         
         return balloon;
     }

However, no matter what's my NPC position, the balloon is always instantiated at Vector3.zero.

For example, this is what the Debug.Log prints:

Debug.Log(transform.position): -4.5, 0.0, 0.0

Debug.Log(gui2dPos): -133.4, 0.0, 0.0

And that is the position of the instantiated NGUI balloon:

P: 0, 0, 0

R: 0, 0, 0

S: 1, 1, 1

Here is the whole scene:

As can be seen, the ballon is in the middle of the scene, not in the same position of the NPC at the stall. I don't know what is wrong. I've been struggling with this problem for months now. :S

Thank you.

PS: As an alternative, I created a UIGrid and added each item there with the following code:

     private GameObject CreateOrderBalloon()
     {
         GameObject balloon = GameObject.Instantiate( _orderPrefab ) as GameObject;
         balloon.GetComponent<GUIOrderBalloon>().SetOrder( _currentOrder );
 
         UIGrid ordersGrid = GameObject.Find("Orders").GetComponent<UIGrid>();
         ordersGrid.AddChild( balloon.transform );
         ordersGrid.Reposition();
 
         balloon.transform.localScale = Vector3.one;
         balloon.transform.localPosition = Vector3.zero;
         balloon.transform.localRotation = Quaternion.identity;
 
         return balloon;
     }

This worked properly. So, I don't think I changing these items positioning somewhere else after the Debug.Log.