The reply function is very inconsistent and weird regarding quoting the post I’m replying to. Sometimes it quotes other posts. Sometimes, it doesn’t display any of the post I clicked the reply link on. And sometimes it works just fine.
The rule for it is basically if you are replying directly under it it won’t quote the previous message, but if you are replying to a message that has others below it, it’ll include it. If you use the big red reply it won’t quote anything.
However if you want to quote a specific part of a previous reply, just highlight the text you want to quote and then hit the quote button that’ll pop up and it’ll import the text into a new reply.
Hope that makes sense.