Input.GetMouseButton only works once

The example from
only reports one mouse click, regardless of the number of times I click the mouse in the game screen.
Same with GetMouseButtonDown.
I’ve confirmed the problem by creating a new project with just an empty object with the sample code as its update method. Just one debug statement is printed per mouse button (left, center, right).
Mouse and trackpad both fail.
I’m running 4.5.2.F1 on a mac.

If you look at your console window, is the “collapse” option enabled? That options tries to combine similar debug messages. I find it causes more confusion than it’s worth.