Sketching sweeps works but picking doesn't ?

[ws]

While I'm quite impressed with Revit doing a fair representation of gothic revival window tracery (see attached), would anyone know why when sweeping the chamfer around the window outlines Revit 2010 will refuse to do it if I pick 3D lines, but will work fine if I sketch the exact same profile by picking lines - identical path as far as I can see.

Picking would be so much quicker - except it will not work...

Weird.