Hi, @laura-harste !
Thanks for posting on the Instructure Community.
I conducted some testing and research on this topic myself, and have noticed that when you create a new announcement, ensure "Allow students to comment" is enabled, and that should automatically take care of the problem.
That being said, are the announcements that you are referring to the scheduled for publication for a future date? If that's the case, no students will have access to create replies at all, or even see the announcement.
Let me know if this helps at all, and thanks for mentioning this in the community!
Noah