We've had a couple of reports of pipe fittings, pipe accessories and in some cases pipes from Revit MEP 2013 just appearing as lines in Navisworks as shown below.
If you are seeing this behavior, there is a straightforward workaround.
In the 3D view in Revit change the Level Of Detail from Medium to Fine
- Export an NWC file from Revit 2013, if you don't have the Navisworks exporters for Revit check this blog.
These pipe fittings should now appear correctly when opened in Navisworks.
A workaround for importing Revit files suggested by Travis Beecher in the comments is to edit settings for the visual graphics in the family itself to show up in medium and fine then when the Revit file is loaded into Navisworks you will see all your fittings.