@FrogrocketStudio: "could fix this as soon as possible" Just so you know, even if Etsy Support or the Help Pages sent you here this is a seller peer to peer forum, and should not be considered a way to contact Etsy, if that is your hope (although Etsy does sometimes read and reply to posts).
Is it possible that the customer started a second (therefore new incoming) thread, such as a Help Request? Have you checked the the applicable CSV file available on your Star Seller page? When you look at the file remember that that the dates / times are in UTC (not your time zone), only the first message of a new incoming thread is included (all that counts for Star Seller), and usually a new incoming thread is not included until after the 24 hour reply window has passed regardless of when you replied. There is also a link to the message so you can confirm which message thread it is.
