NWC files not updating

Hi there, I have been having problems with NWC files not updating.The problem seems to be localised to cad files created in Bentley plant and this does not happen to every file that is created in the program.The file is visible in dwg. format and can be fully rendered,but navisworks does not desplay the geometry at all.The funny thing is the file is listed in the selection tree.
I have found that deleting the NWC of the file in question allows for a new NWC to be created but I don't want to do this everytime a dwg is saved!
Any ideas

Have you installed the object enablers (available free from Bentley) for AutoCAD/Navisworks? You may need to install these before the NWC can update.

Open navisworks and goto TOOLS->GLOBAL OPTIONS->MODAL->PERFORMANCE
and make sure that "Close NWC/NWD files on load" is checked

if navis is not told to close these files after loading them then the exporter is not capable of overwriting the old file.