Are they originally drawn as very thin polylines
with a wider lineweight shown in the Layer Manager or by object? If so,
then the line will default to a zero lineweight because the line cannot be wider
then the given polyline width, and if you are plotting it, the plotter pen
cannot draw the polyline to its width if the pen itself is wider than the
polyline width, so AutoCAD then defaults it to a zero lineweight for both the
color or object lineweight and the polyline width.
Actually everything is drawn using the "Solid", "Arc", and "Circle" commands. No "Pline". This is a 3D drawing using the thickness property in the "elev" command. I can't figure out how to increase the line thickness using line weight or linetype scale. Nothing works.