Cannot update 'Manager Judgment'

When a user is attempting to edit the 'Manager Judgment' column in Forecasting, the field is appearing as locked (for all deals, not just the occasional one).  I have checked and.....

•  'Manager judgments' is ticked in Forecasts Settings under 'Enable Adjustments and Judgments'
• The deal owner is a direct report of the forecaster, under both the Role Hierarchy and the Forecast Hierarchy

In a partial copy sandbox, the field is editable under the same conditions.  It's just not editable in production.  Any ideas why this might be?