function keys to increase brightness don't work

I have a MacBook Air purchased in 2014 - I have just noticed that the function keys F1 and F2 do not work, nor does F5 and F6 - I am trying to increase brightness... I have tried Function keys, Option, Command and Control - nothing works. I have a fully updated system. i can't recall this happening before - is it a bug? Any assistance is appreciated.

I went here - but while it's a great list, it did not provide help to solve the problem - http://guides.macrumors.com/Keyboard_shortcuts

Randy