We are working with Apple to help them fix that FCP X bug. The problem is caused by a bug in FCP X (10.0.4): FCPX does not provide all frames requires to conduct temporal processing. in non-main timeline in an event (not project but event), the filter may not work.With FCPX: if the clip where Neat Video is applied is located
There are some known issues with Final Cut Pro X
This is perfect for DSLR material that has been shot with a high ISO rating, although when reducing noise it won't add detail. If you haven't come across the Neat Video noise reduction plugin before, it does a cracking job of removing noise by analysing patterns in the video.
Neat Video plug-in for FCP now also supports FCP X