<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by cigarman:
Delta also capacity controls international upgrades. Only a set number per flight. THEY NEVER RELEASE MORE THAN THAT NUMBER. Even if the seats fly empty. YES really.</font>
Continental does so too. I had many cases where I was waitlisted for BF upgrade that never cleared, and BF was 1/2 empty on the flight.
This will usualy happen when coach is less than 3/4 full.