I always have had this problem and it used to work with resetting SMC, but not anymore. I've been Googling this for a long time and yet to find a way that would work for me.
When I press the corresponding keys, I get the backlight icon we see in macOS but with a disable icon underneath it. Backlight is working fine in macOS.
Things I've tried on top of my head from various people hinting it:
- Killing its process and starting it with Admin privileges.
- Since I've just installed Windows and Bootcamp, it doesn't need updating, is at 6.0.
- I'm using United Sates (Apple Keyboard).
- Haven't installed Power Plan Assistant.
- In Windows Mobility Center there is no Keyboard enry.
- No Unknown Device in Device Manager.
- I have Light Sensor in Device Manager.
- Covering light sensor does not change anything.
I'm using Mid-2012 MBP (Non-removable battery) using Windows 10 (Build 1089 Redstone 5).