NGUI : UIPanel can't show

I am using this code

 void OnClick(){
     GameObject panel2  = GameObject.Find("Panel-MainMenu");
     NGUITools.SetActive(panel2,true);       
     GameObject panel1  = GameObject.Find("Panel-Title");         
     NGUITools.SetActive(panel1,false);
 }

OnClick function is attach to a button which is in Panel-Title. When I pressed the Button, Panel-Title is hiding but Panel-MainMenu is not showing.

 >UI Root
    >Camera
      >Anchor
        >Panel-MainMenu (Disabled)
        >Panel-Title    (Enabeld)