Problem:
A fairly new Surface Pro 4, all of a sudden, locked the brightness settings and no matter what I tried either with software or the shortcut keys on the keyboard (Fn – Del and Fn – Backspace) wouldn’t work. Was wondering that the auto brightness settings might be keeping me from adjusting the brightness but I still couldn’t after turning it off.
Solution:
Looks like this issue, for me, was a driver problem. I followed the instructions in the link below and I was able to adjust the brightness to my hearts content.
http://www.redmondpie.com/fix-windows-10-brightness-control-not-working-issue-heres-how-guide/