Create Cut Fill Volume incomplete

Question

I'm using the create cut fill volume tool which picks up the whole design model against the existing terrain except one triangular looking area:

I even tried swapping out some of the corridors: 
 
 Any ideas what type of situations cause this? It's a simple model and the existing terrain covers a much larger area than the design so overlap shouldn't be the issue. Also can't see any particularly interesting features in the existing terrain in this area. Tried it in the original topo this complex terrain was based on, no difference. 
 
 Thanks, 
 
 Ken

Chris Andrew · Answer

Only workaround I have for you Ken. Is to turn on Top and Bottom Meshes and use either of these (depends what your trying to achieve) and the existing terrain to generate Closed Meshes manually. Alternatively, building milling/overlay components into your template to do the same.

Chris Andrew · Answer

Make sure all parts of the complex terrain have a feature definition assigned which has the volume option set to existing