Hi Heiner, yes re: non snapping - you must have been referring to center object snap on those ones. A closed and planar polyline curve will have a center snap at the centroid so that you can snap on to the center of things like square or rectangle polygons and things like that. If the curve is not planar that particular snap point won't be present though.

So that's probably just another side effect of some of the polygons being non-planar.

- Michael