Problems cutting through a NURBS surface

I am trying to create a series of lines by cutting a Nurbs Surface with a series of horizontal planes at different levels. At the minute I’m doing this whole process in Python but the cut lines are not cutting all the way through the Nurbs Surface for some reason. The problem is hopefully obvious in the attached image.

Has anyone ever seen a problem like this or can anyone give me any advise on how I might debug this. I have moved the cut planes and the Nurbs surface and not managed to get a different result yet.