If you copied group-restricted discussions from a previous iteration of your D2L course, the discussions may not be properly assigned to groups.
In order to assign discussion topics to groups, you follow the instructions found here.
While you are still able to use the “old” group-restricted discussion format (in which each group is assigned their own discussion topic), a new method is now available. The new method is highlighted in this post, and the new format will allow you to link a grade item to a group-restricted discussion topic that organizes groups by separate discussion threads.
You can find instructions for setting up groups and group-restricted discussions, via the new method, here.