I have a customer using Umbraco 6.1.3 on a load balanced environment and, in general, the site is working fine.  However, when you try to use the 'Publish At' feature, it doesn't seem to fire.  The timeslot passes by and the page remains in a saved state but the changes don't get published.  The date entered into the 'Publish At' field doesn't get cleared out either.

I've checked the log4net log files and the umbracoLogs table to see if any errors were being reported but nothing is shown.  I've also opened up the source for the project to work out what is being called and simulating the selection query via SQL Management Studio and this returns the content I'm expecting.  This seems to suggest that the process to kick the publishing off isn't getting called at all.

Another interesting aspect is that we have a UAT server setup for this client that is exaclty the same in terms of server specs and configuration (and, again, load balanced) but the 'Publish At' feature works perfectly on this setup.

Is there a simple configuration setting somewhere that enables/disables this process somewhere?

Any help much appreciated!