Hello @AlisonRoss
Thank you for contacting the Instructure Community. We're sorry to hear you've been experiencing issues with students replying to your discussions. That is odd and it could be a few possibilities. Firstly, check the discussion settings - ensure that you allow replies. Do you require students to reply before seeing all of the other replies? Secondly, check the due date - if it was in an old grading period for some reason that could be closed to students. Are they able to submit other assignments without issues? Third, it could be a school setting or possibly a SIS date setting if you get your students synced over using SIS. There are other things we can check as well but in a situation like this, we really need to see specifics and have the ability to dive into the settings for the course.
Because of the complexity of the issue, and the concerns for privacy, it would be best if you contacted Canvas Support directly to pinpoint the source of the issue and provide a resolution:
How do I contact Canvas Support? - Instructure Community