Synchronize clips suddenly not working

I regularly use Synchronize Clips to sync separately recorded audio and video. It's always worked perfectly. I always choose 'Use audio for synchroization' and never have any issues.

Since moving to High Sierra, it simply no longer works.

I've even tried re-syncing clips I'd synced previously - and they fail.

Anyone else having this issue? I'm running 10.13.1 and FCPX 10.3.4

Thanks!