I had the same problem yesterday after updating to Mavericks and installing 10.1. Skimmer not working, no snapping among other things. Trashing prefs didn’t work neither did deleting and re-installing FCPX.
FCPX worked fine on a new user account so I was on the verge of a complete re-install of the whole machine. I then tried Digital Rebellion’s Pro Maintenance Tools ‘Housekeeper’ app. Performed a comprehensive clean of all ProApps, emptied the trash, restarted and all now works perfectly.
Hope this works you.
Cheers
James