My guess is that the file loaded to the machine has issues - can you see the linework on screen - if yes then whether it is 2D or 3D you should be able to select it - if you are seeing a lot of Triangles on screen, even though the lines may look like they are there (Triangles show the Linework patterns also as they were formed from the lines most likely) then it is a File Issue. Failing that I think this is a question for the Machine guys - Assume you are running Earthworks.
If you want 3D Lines on the Machine then you need to output the Design as 3D Linework from TBC - however if you only need 2D lines for Guidance (Horizontal Only) then you should send them out as a Design Map associated with the 3D Surface to get what you need. It could be as simple as you selected the wrong option - the 3D Linework is typically for Section Lines that you can chase with an Excavator - is that what you are trying to do?
I will forward this link to the Machine Guys to get you a response
Alan