And, as I've posted before, quite a number of times I've had exactly the same problem but the opposite way (I'm a NW Platinum so I've called NW to upgrade me on CO flights only to find that I didn't really have an upgraded seat when I arrived at the airport).

What I've been told is that the two systems are very different so even though the CO reservations folks have access to the NW reservations system and vice versa, they often don't complete the process correctly. Neither system is particularly easy.