Some courses have a lot of students groups. The groups are used for communication. Students create an announcement or a discussion topic and expect that other students can reply. This is not the case.
Students can only reply if the student that did create the announcement goes back to the announcement home page, and allow comments in the announcement settings.
It is very difficult to explain this to students. There should be one option at course level, or at group set level that allows all students to reply to all announcements.