maybe put the screen closer to the keyboard? i know it sounds weird but sometimes when the screen on my mid 2010 is far from the keyboard, it wont let me turn on the keyboard lights
I just now did the (press ctrl + option + shift + power button) and that seemed to work. After
rebooting, I hit the F8 key and all is well now. I don't understand why I had to do that though.
Could it be you were in a bright area? I know when mine is in a bright area it shuts the keyboard backlight off and give me the little access denied symbol if I try to change the brightness. I dont think there is a way to shut this off using the system, but I did find apps that can shut it off.