Ah, OK. Fair enough. I've been pretty lucky with at-gate upgrades in the past, but this hadn't happened before.
I found where I read that the upgrades on these routes work differently. Re-reading it though, they do mention that advance upgrades do happen occasionally...
http://thepointsguy.com/2012/07/tran...-series-delta/
Elite Benefits
While this route is eligible for Medallion upgrades, they don’t clear in advance like most other Delta flights. Delta normally uses an automatic system to process Medallion upgrades – if seats are available for upgrades (some algorithm which only gives out as many seats as Delta thinks they won’t sell) then Diamonds get them 5 days out (before Platinums), Platinums at 5 days, Golds at 3 days out and Silvers at 1 day in advance.
For the JFK-LAX/SFO routes, however, Delta does not release any advance upgrade space. I think in all the time I’ve been flying these routes, there have been only a handful of reports of any advanced upgrades.