Had same problem when inserting hydraulic ram into assembly drawing. Barrel of ram disapeared when rendering or copy to lines. The barrel was a solid subtraction consisting of a sweep with an extrude subtracted for the ports. As port detail not required in assy drg, ungrouped solid subtraction and deleted extrude for port holes. Was then able to render and copy to
lines.
Vectorworks 10.5.1 mechanical , OS Windows 2000 Pro, Pentium III 800Mhz, 256 ram