iPhone Trade In Credit Not Received

I traded in three iPhones a couple of months ago through an Xfinity store.  Two of the phones I mailed back, I received credit for.  The third, the store took back at the store to process, but I have not received credit for that phone.  I have not received any message from the Xfinity or the store about this phone.  Best way to check on the status of the credit?