Not being able to find a reason why some of the wall lining types show in view but are not printing to a PDF.
Just to make the most of everyone's time, I have already tried:
Raster processing
not really an option due to the extreme low quality of produced PDFs (and yes, I tried the "Presentation" mode)
we need to use the Vector processing
isolating the wall instances in view
they print ok in Vector processing. but then everything else is not showing
Joining walls
Not effect. Still would not show.
Found articles on internet
https://forums.autodesk.com/t5/revit-architecture-forum/missing-elements-on-the-printed-pdf-with-vec...
https://www.revitcity.com/forums.php?action=viewthread&thread_id=27426
https://www.revitforum.org/architecture-general-revit-questions/34407-walls-disappear-pdf.html

Assistant BIM/CAD Manager
Manchester, UK