My MacBook Pro doesn't adjust the brightness when I hit the brightness controls on my keyboard (F5/F6). I've tried turning off automatic brightness to no avail. How do I use the function printed on the function keys?
According to OS X Daily, it's because your light sensor is detecting that you are in too bright of an environment to use the backlit keyboard. Try moving to a darker area, or covering your light sensor to see if it then lets you turn it on.