Ask HN: What determines whether I can reply?

I've noticed that in some cases I can't reply to comments, and in others I can. It's not even a case of not being able to respond to comments I've downvoted, or to comments that belong to people I've downvoted elsewhere, or comments within a particular subthread where I've downvoted someone's comment. I don't see the pattern, and it's frustrating to not be able to reply to someone who is engaged in discussion with me. What's the rule?