Is there anything I can do to lock in a seat reservation? I have long flights (6 hours in coach) to ANC and I want my seat selections to stick.

Every day or 2 my seats are moved from window/aisle to a middle seat. Most times my previous seat remains open so I just re-select it. This morning my old seat was now taken but there was one non-middle seat open so I grabbed it. I messaged the Delta Assistant but she had no explanation, and could only put in notes in the reservation, not lock the seats.

The ticket is paid for and there is a receipt on file. Outbound I specifically bought an upcharge to comfort to get out of a middle seat, so it is especially irritating to keep losing this paid extra for seat. Check in is tomorrow and I am afraid I will lose my seat again between now and check in.