Original machines: The milkbutton does not light up on my Lattissima One, why?

First make sure that milkcontainer is connected. The milkbutton will only light up when you attach the milkfunction.
Whilst attached, the sensor on the machine panel will register the fluid in the milktank, and light up the milk button.
If the button does not light up then first check the milk you are using and that you have filled the tank with enough milk. 

Make sure to use milk with a high fat content as no- or low fat milk might not be registered by the sensor.