I replied to a review again and nothing new appeared

The editor does not post the same reply from you under the same review twice. Before it posts, it looks under the review for that reply, links and lists included, and the message says so: "1 review already has this reply from you, so it was not posted again." Your first reply counts in any state but the Trash. If you cannot see it on the product page, it may be waiting for moderation or marked as spam: in the Reviews grid, set Type to Replies in Filters (and Status to Spam if it is not listed) to find it. To add something, reply again with different words. A review that already has your reply keeps its status, so one waiting for moderation stays waiting until you approve it.