1. Right-click on the appropriate Group Header section
  2. Select Change Group
  3. Click on the Specified Order tab

    Change Group Options - Specified Order tab
     
  4. If the value expected is not on the list then select the name name of the missing group from the drop-down list

    Change Group Options - Specified Order list of values
     
  5. If the value expected is on the list then click Edit and add the criteria

    Change Group Options - New Group
     
  6. Click OK, then OK again to save changes
  7. Also, groups that are based on formulas might not group correctly because there is an issue with the formula itself so check the formula as well.