In a course where separate groups have been set up, the choice of Simple Single Discussion Forum appears to allow groups (there is a select group drop-down when posting by instructor). However, it doesn't allow this so students do not have a reply button.
This has been a source of immense confusion and frustration.
Suggested improvement (easy I'd do it myself if I knew where to find the text)- add text to the help section to highlight this fact.
Testing - when clicking the forum help button, this information is displayed.