If you did not figure it out yet, there is a major flaw in the NWA upgrade process. If you go on the waitlist on-line, your chances are almost 0% for an upgrade. The problem is that if a seat opens from the time you complete your on-line checking process, the system does not go to the waitlist to provide the seat for the first passenger on the list nor does it save the seat till the boarding process. Instead it remains open for the next person that signs in or a passenger checking in at the airport. Just a couple of months ago, I checked in on-line from DTW to SFO with a Y fare (Platinum) and was put on the waitlist. When I got to the gate, my colleague had also a Y fare (Silver), I asked him and to my shocking surprise he got an upgrade when he checked in at the airport. I immediately called the Platinum line to complain, their explanation proved nothing more than the flaw I explained above.

Continental does not have this glitch. I was upgraded on several occasions after the on-line checking process before I got to the airport.

My guess, to fix this problem it requires a major software upgrade that NWA is not ready to spend the $ to fix. So in the meantime, do not complete the on-line checking, just keep trying and you might get lucky and sneak in.