In a Q&A forum, students post before seeing other students' postings. Once they have posted, they should be able to reply to postings, but only to those postings that were made by people in the same group. Is it possible that your students are trying to reply to postings made by students in a different group?
I'm curious to know why you're using a Q&A forum. I have seen other users suggest that Q&A it is an alternative to Forum for general use when you want to avoid overriding Student role to permit starting a discussion. They're wrong.
Forum
Groups in Q&A forum
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