While I've never owned, or used as Sony laptop, I have had a similar experience with an IBM Thinkpad.
Certain function keys would not work properly, or at all; brightness was one of them. Another was the function key for getting out of sleep mode.
After upgrading the BIOS, this was resolved.
Hope this is the case for you.
Mick
Complacency is the rigor mortis of the soul.