For example, tonight I flew DL 1791 from ATL to Sarasota on a U-class (discounted) ticket. As of yesterday, there were 5 or 6 unclaimed seats in first class, and I was listed for an upgrade. The upgrade didn't clear, so when I arrived at the boarding gate in ATL immediately prior to boarding, I checked the posted upgrade waitlist. At that point, all the first class seats had been filled except for one, and there were still 9 people in queue awaiting upgrades. I was #9 of the 9.
I too just had a similar experience that was hard to understand....
I am both DL GM and NW Plat. So after the recipricol upgrades were announced, I started to use mt NW Plat on my reservations, thereby being recognized as at the same level as DL PM for upgrades... or so I thought.
After making this switch, I have been upgraded on all of my last 5 flights, on the PM-heavy ATL-FLL or ATL-PBI Thursday night route. As a Gold I had not been upgraded since flying this route (started in September 2008) every week, except for the Wed. before Thanksgiving.
But this past Monday, I had not been upgraded up until flight time, although there were 5 F seats 1-hour before. I did OLCI on my way to the airport and printed my BP at the kiosk. I made my way to the gate and saw I was #9 out of 9 on the upgrade list. I probably should have quizzed the GA, but I was too tired and at the moment indifferent the upgrade. Also, once you check-in, it is impossible to change your FF # on the entire reservation.
As globaldude points out, there is no way there could not have been other GM, SM, etc. on that flight. I know for a fact that were several GM who fly with me every Monday morning, and were ahead of me on the upgrade list.
My conclusion is that Delta's upgrade system is properly allocating upgrades to NW Plats at the upgrade windows, but once control is handed over to airport and gate, for whatever reason you are dropped behind others.