I haven't done a lot with NURBS, but here are some things to keep in mind.
The object must be convertible to a single Editable Poly. It can be NURBS, but the thing that the sculptmap is generated from is always an Editable Poly. Prim Composer clones the object and converts it to an Editable Poly before generating the sculptmap.
The UV map on Map Channel 1 should be a regular grid with mostly perpendicular lines in it. I mention this because the NURBS tools in 3ds Max like to warp the UV map according to the contour of the model. If the UV map is warped, it can cause distortions in the sculptmap. If this is happening, there is a way to prevent it. See the tutorial Creating a nurbs.max file for 3ds Max 7/8 on the Video Tutorials Page for some hints. The steps to prevent warping of the UV map are described there. It is very simple, but I can't remember exactly what it is ATM.
Hope this helps, but I'm not sure if these comments are relevant or not.