Why don't I get notifications when people reply to my comments?
It's especially annoying if I have asked a question in a sentence discussion.
Either you don't follow the discussion, there was lately an issue that it wasn't marked automatically, or you switched off the messages in the settings. Or it is a recent bug.
Just in case your notification settings are correct and you still do not receive notifications for new comments on followed discussions: change your e-mail-address temporarily for some hours, change it back and verify it again. That did the trick for some others.