Originally Posted by Kettering Northants QC
I think sometimes no matter how much money, miles, status you have in your pocket or what fare class you're booked in they just don't do upgrades.
Yes it's all a bit mysterious, but sometimes upgrading with miles at the checkin works. I got upgraded for miles from PE to UC on a SFO-LHR once, and they didn't even remember to deduct the miles! However, they also didn't give me any flown miles or tier points for that flight, and claiming them afterwards did not work.
But at other times they do the whole calculation and then say "oh but the computer isn't letting me do that right now" or suddenly decide at the last minute (after completing the paperwork and signing the miles away) that you have the wrong kind of ticket. I just always give it a try.