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Are you attempting to adjust the brightness with the Intel HD Graphics Control Panel?
You can access it by right clicking on your desktop and clicking "Graphics Properties". The click the "Display" tile, and then the "Color Settings" tab. From there you can adjust the Brightness slider. Let me know if it works.
I am having the same issue on a Lenovo Thinkpad W540 running Windows 8.1. I updated the Intel HD Graphics 4600 driver to the latest on the site (10.18.14.4080) and still no luck. Going through Charms -> Settings -> Brightness doesn't work nor does the hardware brightness buttons. I am however able to adjust it using the Intel HD Graphics Control Panel.