Missing Labels in Site - Joined tables

I have an MXD that has a polygon feature class joined to a separate table.  The feature class is symbolized and labelled from values in the joined table and each works fine in the MXD.  When the Map Service is published from the MXD to ArcGIS Server and consumed by GeoCortex Essential 3.14, the symbology for the layer is fine, but the labels are missing.  Both the symbology and the labelling worked fine in the GeoCortex Site for several months up until I re-published the map service recently.  This is strange because:

1. When the map service is published to our non-load balanced test ArcGIS Server environment, the labels work fine for the test  GeoCortex site (running on same test server).

2. When the map service is published to our non-load balanced test ArcGIS Server environment, the labels work fine for the production GeoCortex site (pointed to the map service on the test server).

3. When the map service is published to the load balanced production  ArcGIS Server environment, the labels do not appear for the production GeoCortex site (pointed to the map service on the production server).

Not sure if the issue is with ArcGIS Server, the load balance software or with GeoCortex Essentials.

Any thoughts you might have on the matter would be greatly appreciated.