You need to read the discussion until the last post for new notifications to come in.
Marking the notification as read is not sufficient. But if you clicked on the notification it should have opened the discussion and marked the discussion as read.
There's a possible issue if a new post was created and then deleted, you might be unable to have the discussion marked as read until the hidden post, and this could impact the ability to send notifications because it will think you still have unread posts.
