Double check you are allowing permission for FCPX to access your camera AND microphone. Strange I know but the first time I went to capture VX footage in FCPX I had no video, no nothing. Diagnosed it to this. I captured from a VX2100 and a random Panasonic capture cam with no issues
System Preferences > Security & Privacy > Privacy Tab > Scroll down to Camera and Microphone and make sure FCP has access to both
If that doesn't help it might be capture cam