Via Help: Parts can be created from:
- Foundation Slabs
- Floors (no shape editing)
- Walls (not stacked or curtain walls)
- Roofs (that do not have ridge lines)
This last item, roofs, is the focus of this post. I find that if a roof ever had a slope, causing a ridge/valley line to appear, it will not activate the Create Parts tool even if the slope no longer exists. If you use the Shape Editing features for the roof it too will "kill" the Create Parts tool. If you use the Reset Shape button Revit will restore the Create Parts tool.
- Revit seems to retain the ridge/valley information whereas it will forget it if you use shape editing instead.