First: (Assuming you closed-out, re-booted, and re-started FCPX) make sure "skimming" is turned on.
If that doesn't do it, then...
Trash your preferences. (Harmless.) Easiest way to do that is to get this tried and true (and free) app:
It will also let you store your preferences for easy recall should you have to "trash" again.
Hope this helps!