If you run the following command: $ xev -event keyboard
..and press the function keys, do you get anything?
For example, pressing F1: KeyPress event, serial 28, synthetic NO, window 0x1600001,
root 0x311, subw 0x0, time 830988, (-7,-1), root:(1179,585),
state 0x0, keycode 67 (keysym 0xffbe, F1), same_screen YES,
XLookupString gives 0 bytes:
XmbLookupString gives 0 bytes:
XFilterEvent returns: False