Surprise! From Apple's TA1099 Knowledge Base article.
You will not be able to successfully export from Final Cut Pro using Compressor if your computer is set up as a Compressor QuickCluster, or a "Virtual Cluster." If you attempt to send an item from Final Cut Pro to Compressor in such a case, some segments in the job may report "Failed" and processing of the batch will not complete successfully.
What can you do?
You can work around this issue by first exporting a self-contained movie from Final Cut Pro. You can then use a QuickCluster in Compressor to transcode that self-contained movie.
Alternately, you can export from Final Cut Pro directly using Compressor, but use a single-service instance of Compressor instead of a QuickCluster.