Sometimes we just screw up. There are so many things that we, as gate agents have to do to get the flight out on time. That doesn't excuse failing to upgrade a customer when a seat is open but it happens. If you believe that you should have been upgraded and a seat in F/Class goes empty on your flight please contact the Customer Service line. That's what they are there for. Flight attendants are not allowed to upgrade unless directed by the gate agent. Just imagine being a hub gate agent, during a major weather issue, having customers rebooked onto your flight. A gate agent can lose control of the upgrades because things are happening too fast and your primary concern is getting the boarding completed and leaving the gate on time.