Input.GetMouseButtonDown(0) always false

I am calling this in a update() and its always false. No idea why I can get any mouse input at all. Anyone know anything about this? All other keys work fine.

try these steps and see if it works:

1- create new scene.

2- create a java script and write this in it :

function Update(){
   if (Input.GetMouseButtonDown(0))
      print ("it works !");
}

3- save the script and attach it to your main camera.

4- play the editor.

5- click left mouse button in the game window and see if anything printed out on console.

Ok, found the bug. My game tab was on its own window and not taking input for any of my mouse clicks. I moved the game tab to the Unity window and it all worked fine. I will submit the bug to unity.