Katherine -
Cross-posting this to Geodatabase since this question is centered on the Utility Network Analyst toolbar in ArcMap with the geometric network and not the Utility Network.
Do I understand that you are not seeing any results returned when running a trace in the geometric network? Are you seeing the same behavior when you return the trace results as a graphic (or as a selection)? Is this behavior seen when working with the same data in a new MXD? What about when you attempt to trace other datasets? I've seen behavior similar to that you reference linked with a specific MXD in the past.
Thanks,
Jon