I am on OS X 10.9.5 and recently (can't remember if this happened during 10.9.4 or not)
FunctionFlip has started to work very inconsistently for me.
Basically I use this to enable the F1-F10 keys for use in terminal (and some of them I set to do stuff like next/prev. tab e.g. in web browser) and I used FunctionFlip to generally enable the volume control keys so that they work globally.
But lately it's just not been "kicking in" so it's hit and miss every time i try to change the volume.