When you press F7 key, you see splash info on the screen, like the picture?
Try to press only F7 key, and combination FN+F7 key.
If you don't see the splash then it's probably a problem of drivers, or of the key itself.
If you see the splash then it can be a problem of the drivers or the backlight...
Do the other function keys work and show their splash? For example F1 cuts sound, F2 decreases volume and F3 raises it?