(Applies only when Tangent to surface is selected under Mold Parameters.)
With Optimize not selected, a curved surface is generated. It is difficult to machine this surface.
With Optimize
selected, generation of the curved surface is blocked, so no surface is
created in that area. Use surface tools such as Planar
Surface 
and Knit
Surface 
on the Mold Tools or Surfaces toolbars to
extend the parting surface into that area.