You probably didnt get the upgrade because MCS was indeed full however maybe that person missed the flight. You have to let the agent at the gate know you would like to be placed on the standby list for MCS and when they close doors 10 min prior they can just reprint your boarding pass if someone didnt show up for their MCS seat. Sometimes you can just tell them you will board early with priority but just have them come move you before they close the aircraft door.

Unfortunately the agent will not know if MCS is available until they are handing paperwork to the crew which is usually 10 min prior to departure. Its up to you if you want to wait up at the boarding area. They will NOT auto move SLV and GLD guest if a MCS seat opens up unless that guest has requested to be placed on the standby list. The agents do not know if you are flying with someone else and do not want to separate. I have also seen SLV and GLD guest reject MCS if there is nothing left but a middle seat and they were comfortable in their window or aisle seat in MCE. Would be a shame if they auto moved you without your permission to a middle MCS seat, and you dont like middle seats and now your old seat has already been given away! Alternativly you can show your boarding pass to the In flight team member and they should let you move no problem. Finally if a SLV guest request standby for the MCS and the GLD guest doesn't it will go to the SLV guest as the GLD guest never requested it.