NGUI (Button) Highlighted Sprite doesn't remain highlighted

I’m currently using Unity NGUI system and having some issue with button highlighted image (Button Script). It works fine when you click on the button and the button will stay highlighted, but the moment you click outside the button,the highlighted button goes away. How do you retain the highlighted state of the button once you click outside the button?

Check the button, it may have entered the “hover” state. Disable mouse in camera settings (in Event Sources)