Transition for buttons do not work before interaction

Hi there,

I'm using Unity for a student project, a RTS game. I have a button in my HUD that change depending on the kind of unit the player can build; the button is only visible when the main unit of the player is selected (otherwise, the image associated to this button is disabled). My problem is that the transition of this particular button does not seem to work until I interact in any way with this button (by moving my pointer over it for example).

When I first launch the game, the button has the "Disabled color", but is not disabled.

After that, when I select my main unit, the button has the "normal color" when it should be disabled.

After I first interact with this button, the button reacts like it should.

Did I miss something, or is this a bug? I'm using Unity 5.