Why am I not getting notified of new discussion comments?
Email Notifications are sent only when you are @mentioned in a comment. To get notified of all comments, ask your admin to enable the discussion notification workflow. Enhanced notification options are coming soon.
Head to Settings > My Notifications to establish your notification preferences. You can also choose to be notified via slack if you are mentioned in a discussion post.

*PRO TIP. In order to ensure that all customers and team members know to use the @ symbol within discussion posts; create an early task and include this information as a best practice.