If you are using an image file as a background, perhaps for tracing a photograph JPG of instruments, check the Z coordinates of various things in the drawing. Also use the "display order" function (on a pull down in my menu layout) to move things up or down above images so they are not hidden. I have a license for an old version of AutoCad and it definitely has some bugs that occasionally cause some drawn objects to not display or plot sometimes even if all layer settings are correct. As a user of AutoCad since version 2.1 in 1984 sometimes I have a drawing that has drawn objects or blocks that do not display or plot properly with no explanation. I usually find a work-around, sometimes "xref-ing" the problem drawing in and drawing in a new drawing. As a last resort try drawing things in paper space. Also try the "audit" function (type it at command line) to fix drawing database errors, purge and "wblock" out parts of the drawing.