Openplant 3D file translated to AutoCAD 3D. The pipes disappear when the operator flat files the file.

I am exporting a OpenPlant model to AutoCAD.  When the AutoCAD operator opens it up and flat files it the pipes disappear while the fittings remain.  I am not an AutoCAD operator.  Is there a setting I can set when I translate it to AutoCAD.  I merge all the reference files into one file.