My only denied upgrade in the last year was IAD-SFO. And this was also on a 757, not one of the Airbii - chosen specifically so I had a better chance of upgrading. There are many threads here on the IAD-LAX/SFO runs, it's probably now one of the toughest domestic upgrades in the system.

Reasons include:
-So many of these flights are on Airbii that even the few flights on larger planes get jammed with people hoping to upgrade.
-The larger flights are often used for connecting int'l traffic with lots of paid C/F traffic).
-And yes, of course, the discounted F fares are also reducing upgradable space.