not receiving device trade in credit

I traded in 3 phones in September and got confirmation that all 3 phones were accepted with the messages "Congrats! Your upgrade on ** line is now complete. You should start receiving your new device discount on your next bill. The discount was only applied to 2 of the phones. I'm still being charged the full monthly cost of the final phone. I worked with support in November, and they told me it would be corrected for the upcoming bill. It has not been corrected.