Not sure what exactly it means but I believe it is
only IV not NT at fault here.
What I've found is that IV doesn't like some parts
for some reason. If you reduce the selection set you are demoting
you'll find that somewhere along the way it works.
There is probably only one or two parts that it
won't or doesn't want to demote... not sure why though. Maybe it has to do
with a part that was created in place (in the assembly) or that used projected
edges or something ??
I never did track down why I just know I could beat
it but changing my selection set of parts and then individually dragging over
the remainder of the parts one at a time... bit of a pain as you loose
constraints that way.