Offset surface not working properly

Rhino
1

I created this surface as a subd, converted to a poly surface and then when I try to offset surface, one face wont offset.

1.PNG
1.PNG1232×868 123 KB

SHELL.3dm (6.1 MB)

Thanks for any input as to why or possible solutions.

2

offset the subd first with solid= yes, then convert to nurbs-

bonus prize, you can adjust the offset surfaces easily in subd if you need variable thickness-

image
image1271×755 174 KB

3

Thanks! I’ll give that a try.