I have 21 message chains for July - September but only 2 are being used to calculate my message response rate.
@SandyStitching: Was the first Message of each of those Message threads received during that period? Keep in mind that the Star Seller metric is NOT all new messages but only the first message of all new incoming threads.
Your current period statistics are for August, September and October.
Only the first message in each new thread requires a response. How many of those 21 threads actually began during the assessment period and were responded to within 24 hrs? If you have older message threads which the buyer keeps responding on, only the original post in the thread counts for SS.