Originally Posted by JMN57

Well documented, if you are a standby passenger, you are behind all confirmed passengers on the upgrade list. If the GA in looking at the PALL list realizes that there's no point in putting you on it and tells you no, the GA is doing you a favor and saying get on-board and get your bags stowed.

If getting on the list is important, make sure to do confirmed same day flight change and get on the list as a confirmed passenger.