When trying to perform a Trace or a Locate operation using ArcFM for Silverlight, or directly on the server, either an error is returned (in the case of performing these tasks on the REST endpoint on the server) or zero results are returned, yet an identify operation returns results.
In almost all cases, the mapservice being used to Trace/Locate does not have an ArcFM Server license. Typically this is because the ArcFMMapServer capability has not been added to the mapservice. (Note, the name of the capability in 10.0 and 10.1 is ArcFMMapServer, but in prior releases, the name may differ slightly.)
Verify that the mapservice has the ArcFMMapServer capability and that ArcFM Server licensing has been setup for the GIS Server.