See if this helps. It is from a similar question Re: Q&A Forum 2.6: No "Reply" link visible to students
Yes....the reply button does not appear...for single simple discussion format when there are separate groups.
We have come up with a workaround for creating the equivalent of a single simple discussion for a course for meta courses...but the same idea applies to any course where you are using separate groups.
It is a bit of effort to set up...but is transparent for the students to use once set up.
Here is a print how to. https://docs.google.com/document/d/1CS6XidQyQOfkPwmm2AstB4oZ7U--n6k0I1Vr6FeUxF8/edit?usp=sharing
and a video we made.