Trouble exporting as Autocad Solids

In the attached file the only items that successfully export to Autocad 2011 as solids are the wheels. The frames just disappear. I’ve had this happen before with polysurfaces that I have edited using control points. Not sure if that’s a cause or a symptom. Any thoughts would be apprciated.
DennisRolling Plexi Screens.3dm (7.3 MB)

Hi Dennis - the frame has a bad surface-(Check command). Use ExtractBadSrf, then Delete, then Cap to close it up again- does that work now?