Yeah so I called an AC agent and she cleared things up for me. When you pay extra to buy a Latitude fare that is "immediately upgradeable", you can only upgrade IF the (small # of) J seats that AC has designated as "upgrade seats" are available. If they have all been taken by other upgraders by the time you attempt an upgrade, you get put on a waitlist. Then AC may (or may not) release additional J seats to be "upgrade eligible". They may try to sell them all until the last minute, and only then, 5 min before pushback, decide to release them, at which point you will be wresting in the waitlist with others who may have priority over you (super-elites, those who paid an even higher Latitude fare because they bought at the last minute, etc etc).

I thought because I was paying extra money for a Latitude fare, that was advertised as "immediately upgradeable", I could actually immediately upgrade.

AC, although you may be technically correct, that there is fine print (somewhere) that explains all of this, I wonder how you think you are affecting your customer's sentiment towards you, when you act like this. What do you think is going through my mind? Am I happy with this? Am I neutral with this? Am I unhappy with this? Do you even care?