FCPX export to Compressor not working?

Never used to be a problem. I'm exporting a film from FCPX timeline (project) out through Compressor. I do a share for FCPX; my Compressor settings have been set up (so I have a pre-set icon). But nothing happens. I look at my progress bar - zero. And the file it creates remains always at zero bytes.

I rebooted FCPX two days ago when this happened ... and got it to work as it should. But no amount of rebooting seems to work now.

I've done this both with Compressor actually open, and not open. In the past it didn't need to be open for the whole process to work.

I'm wondering if anyone else has been having this issue and what might be causing it?

Ben