I can confirm that number of years as a plat has nothing to do with it. from what I can tell, it is all about how far in advance you buy the ticket (all things being equal). I have been Plat 6 years and early this year I had a bunch of last minute trips. In every case I where I bought the ticket inside a 2 week period I did not get the upgrade, or was one of the last on the list to clear at the gate. These are Plat intense routes also. On one flight from DTW to SFO I paid a very high fare ($900) because I bought it inside 2 weeks. I did not get the upgrade. The next week, same flight, same day I had a ticket that was booked 4 weeks out and I got the EUA at 7 days. I think when you go on the "list" you keep your assigned spot. So if you were the first on the list, you will be the first to clear regarless of fare, but in rank order of status.

1. Status
2. When purchased
3. fare

Bottom line; I buy my tickets way in advance, save lots of $$ and get the upgrade most times. If I have to do a last minute, I research less popular routes and times to have th best chance. I always check in at 23:58 but I am not sure that does anything...but just in case