These groups must be manually rebuilt before they show all of the constituents that responded to the survey question. To rebuild such a group:
  1. Find and "Edit" the survey that created the group.
  2. Click on "Include Questions" on the left.
  3. Click on "Manage Survey Response Groups" on the bottom left.
  4. Click on "Refresh Group".
  5. On the right side in the list of group(s) it will probably say "Processing" in light grey. Click on "Refresh Page" until that is gone and the date in the same column is recent.