Unfortunately, Carbide Create is still pretty limited in which DXF constructs it can import.
it may help to use the OVERKILL command to eliminate any overlapping or intersecting paths before exporting from AUTOCAD
if lines have different elevations, use FLATTEN 
PLINES are intrinsically joined, and should be used where appropriate 
AutoCAD 2000 DXF format (model geometry only, base model scaling) export from the desired face (not isometric view)
Other options include opening and re-saving the files — I usually end up opening in LibreCAD, saving as MakerCAM SVG, then fixing in either Inkscape or some other tool.
Please send any problem files in to firstname.lastname@example.org and we’ll hopefully get them added to the test suite and importing properly.