Illuminosity masking with Alt + left click not working

Hi,

New to Photoshop and I'm trying to learn how to use the illuminosity masking technique on my photos.

When I have applied the layer, I hit ALT + left click on my mouse to show the mask but it does nothing.

my mouse is plugged in and the usb socket is functioning fine. I can see the mask in my layers panel. I also tried hitting ALT + left click on my laptop's touchpad. same non response. Is there another setting somewhere I need to activate first?

my workflow was add brightness contrast adjustment layer / image / apply image / went with standard settings there then clicked ok. I see in my layers panel the brightness and contrast adjustment layer and the layermask thumbnail showing but nothing happens when clicking ALT + left click to show the illuminosity layer. 

Any help is much appreciated, thank you