I Quote from another forum, so i have no idea if this works but
The steps are simple:
1. Using Navisworks Simulate 2011 or newer, open the file(s) to be converted.
2. Select the Output tab and use the FBX command in the Export Scene panel.
3. Set the settings shown in the FBX Options dialog. I have had the best success with using the FBX File Version FBX201000. Turning off the Textures, Lights and Cameras will speed up the process (you won’t typically use those) and don’t use Polygon Limiting or you will lose geometry.
4. Save the FBX file to your C: drive (this improves performance).
Did you get to bottom of this? We are hitting the same issue - and its causing us some real problems.