I have the same issue with IGS imported curves. Creating nurbs curves from scratch and converting those works fine.
I have no solution.
Hi,
Do you think your Bezier curves are not converting to nurbs curves because they are still called something like "bezier1" after you convert them, or is there some other reason? (The name won't change automatically.)
Try this: Create a Bezier curve, then convert your curve to a nurbs curve. Right click on the curve and select "control vertex." Note that your Bezier "handles" are now gone. The curve is more like an EP curve now.
You can actually loft nurbs surfaces using Bezier curves without converting them, because Bezier curves are considered a subset of nurbs curves.
Hope this makes sense...