Problem:
Valley Subzone not working as expected. See image below.

Solution:
Subzone is too narrow.  See how it is just inside the points.

Turn on grid points by Right Click on Proposed Topography in Project Explorer > Properties > Tick Show Points.

Make sure the subzone width is at least 1.5x wider than the calculation grid point spacing to ensure at least one grid point is covered by the subzone for each row/column of the grid, no matter what angle the zone runs relative to the grid (eg 45 deg). See image below.