Okay so I have a simple sphere split into two shells. Usually if I select the shells edge border and go to Edit > Detach, it simply splits the shells in two.

For some reason though some meshes do not. What are the rules for detaching an edge? I read on Mayas documentation, from what I read, I should be able to simply select and edge loop and detach.

Its all in tris, if that matters? I have no idea why some would and would not detach at the edge.