As sugested it appears the object created with the loft is zero thickness shell of the sides only, as defined by the open top and bottom 3D polylines. Strangely after exploding it showed as a solid, but using Modify>Modify 3D Objects>Create Solid>From Surface would suggest that it was an open shell.
After some experimenting it turns out the problem is that my top sloping 3D polyline was not in one plane, there is twist as the top of the ramp due to a slight S shape. I re-made two lofts for different parts of the ramp approximating the slope twist with two different top planer polylines. The resulting loft was also a zero thickness surface, but it converted to a solid using Modify>Modify 3D Objects>Create Solid>From Surface.
Thanks again, much appreciated