I have just experienced this problem in fcpx 10.1
I edited a clip and was able to paste the settings, but when I tried to copy another set of settings from a different clip the paste feature stopped working. I went back to the first clip and attempted to copy its settings to a new clip and FCPX did work with that clip. It was only the settings in my second clip that FCPX wasn't liking.
The difference between the two clips was the Pan Mode setting in the Volume and Pan dialog box. When the Pan Mode was set to Dialog, FCPX would not allow me to copy any of the settings. When I changed it back to None, FCPX's paste settings feature was restored.
Solution for me: Set the pan mode to None, copy the settings and paste to all desired clips. Then change the Pan Mode back to my desired Dialog setting with ALL clips selected to apply the setting to all of the clips.
It is possible that there are other settings FCPX will not paste, so if you are experiencing this problem you simply need to isolate which one is causing your problem and then apply that setting manually.
Hope that works for you.