a lot of autocad will not display 3d p lines with linetypes. tbc does. one solution is to flatten all your linework down to 0 or a suitable elevation as long as they are 2d then it should display correctly.
if you are using civil 3d then LINETYPE3DPLINEON will hopefully work for you.
if this does not solve it, then there could be other linetype issues. pretty sure that if the linetype does not exist in what ever autocad template you are working in then it should be imported. but lets say you already have that linetype in your autocad template, then the autocad one will override the import. pretty sure on this, this is what currently happens on our out of date tbc linetypes that have been fixed in civil.