I've upgraded 2 seats DOH-LHR in the past, and it was rejected over and over and over. I just stuck to my guns and repeatedly went to the QA office every time and resubmitted the request until eventually I got the seat.

Why is it so difficult? No idea.

Is it impossibly? No, but they seem to want you to think it is.

I don't claim to have all the answers, but I wouldn't just submit something online and wait for it to be processed. You need to pick up the phone, or better yet visit an office, and speak to an actual human.