I am 3 for 3 using the NW upgrade certs on AS this year, each time on a NW ticket. I just called around the 24 hour mark and got the upgrade. On one trip, however, I had the same experience as the OP, but I just called back, got a different rep and got the upgrade.

I had basically no wait on their phone line, the planes were very clean, Board Rooms were great, on-board service was top-notch and flights were all on time. On the minus side, the F seats seemed cramped (I was on the 734 for all flights) and food offerings were meager. Lots of discussion about these recent food downgrades on the AS board.