Publish failure

We have a Windchill 9.1 installation that has sporadic failures to publish previews:

Jan 18, 2011 8:52:45 AM:ProE Drawing Conversion
Jan 18, 2011 8:52:45 AM:Error, did not find available worker of type PROE

It has worked, sometimes well. Sometimes dropping just one in a series.

Any hints on what needs tweaking?