Why the students in separate groups are not able to post reply to a forum discussion started by the teacher?

When using groups, teachers are given the option of adding a new discussion topic for all participants or for a selected group.To enable students in separate groups to reply to a discussion, the teacher must copy and paste their discussion topic for each separate group, selecting each group from the dropdown menu at the top left of the forum page before clicking the "Add a new discussion topic" button.