This hint is probably a DOH! for many but I spent way to much time figuring this out so I am sharing it feeling stupid.
I was having recurring problems with DXF exports and imports from SU.
Sometimes they worked and other times they did not.
Turns out that the object you want to export must be flat on the x axis to export anything but a line (for the x axis).
In the picture below the vertical object (even if you select it face on) will not export correctly but both of those on the right will.
DOH!

