Hello there. Check two things: (1) is it a News forum - the one which appears by default at the top or every new course and to which users cannot post (it is just for announcements) or is it (2) set up for separate groups and you haven't posted in each group for them to be able to respond? See #1 and #10 in Forum FAQ
Teaching with Moodle
Forum non accessible to users
This discussion has been locked so you can no longer reply to it.