Ensure that the Steam overlay is on as disabling this will stop the F12 function from working. Failing that... I've found that when this happens to me (and it happens regularly), close down the steam client completely and then relaunch using Run as Administrator. Also, make sure that if you're function keys need to be locked that you've done that. This is something that I also need to do on mine, otherwise I just end up turning my volume up when I hit F12. Hope that helps.