I just struggled with this for most of the morning so decided to share it here.
If you are exporting from a 3d model, make sure that your orientation is correct. It will export everything visible from the 'Top' Camera angle within SU. To get to that view click Camera > Standard views > Top.
If your 2d image is at a 90degree angle to this plane like mine was it will only export the top edge, which is a single line.
Hope this helps somebody out!
Thanks for a great tool.