Might be worth looking at this:
https://help.sketchup.com/en/sketchup/importing-and-exporting-cad-files
There is a list of supported and unsupported CAD entities.
This is still a problem in version Sketchup 2020(20.1.228) and Autocad for Mac 2020. I had to make a lisp routine to re-save the drawings. Also there is no way to have Sketchup just export all on layer 0 when using section fill.
(defun c:zer ()(c:zx)(COMMAND “change” “all” “” “p” “la” “0” “” “qsave” “close”)(princ))
Please fix this Sketchup team! Been a problem for years now. Every change made I have to re-run this on the new DWG made. I am saving down to Autocad version 12 to not have the NLS warnings…Please give some love to the people paying for your product!