I'm posting this message hoping that it will help other people who may encounter the same problem I have been experiencing.
I'm exporting entities to a shape file using the Map 3D Export command (mapexport) but no entities were exported to the shape file.
I found that if I included the EHANDLE property in the list of attributes to export no entities were exported, but if I removed the EHANDLE property all entities were exported.
Upon further review I found the problem to be tied to the length of the EHANDLE string property that is automatically created in the shape file. For some reason, Map is setting the string length to 10 despite the fact that an entity handle can be up to 16 characters. If any entities in your drawing have a handle greater than 10 characters they will be excluded from the export. (To see the handle of an entity you must use the LIST command. They do not show up in the properties window)
To resolve this problem you must reset the handles on your entities within the drawing. The only way I know to do this is to create a new drawing and insert the old drawing as an exploded block.