Trade-In Credit Never Applied

I switched to AT&T after being promised trade-in credit for 2 devices. Both devices meet all the terms & conditions for the credits: devices are paid off, eligible device types, both devices function perfectly, etc. When I look on the "check the status of your trade-in" link at AT&T, they both say they have been received and the credits are being applied to my bill. However, I am only receiving one trade-in credit for one of the devices. The other device credit has never been applied. I have contacted customer service every month about this issue and continuously been told I need to wait. Last month, the representative told me that it would be applied to my next bill and include a back-dated credit for the months I had not received it. Lo and behold, my new bill gets posted today. Not only is there no credit applied for the second device, but my bill has actually increased $9. I was chatting with a supervisor online when he suddenly disappeared from the chat and never came back. He told me I would have to speak with the trade-in department. I started a new chat and was told I was being transferred to another supervisor, and then I was placed in a queue that never went anywhere. I called and was told there was no supervisor available right now. Nobody seems to be able to explain to me why I am not getting a credit for both trade-in devices. Even if it weren't eligible for some reason for the monthly credit, AT&T claims they will at least credit you for the market value of the device. I have received nothing.