I have not experienced the situation you had specifically, but I had an issue where I was passed over for an upgrade (when being #1 on the list for 1 seat) due to not being at the gate when my name was called. I arrived to the gate around T-30 min. I challenged the agent and she admitted to skipping me because I wasn't at the gate, and that it was too late—too bad so sad. I later wrote into Delta and asked them to review and clarify if the Delta policy requires the passenger to be at the gate when boarding starts and their name called in order to receive an upgrade (I was at the nearby SkyClub). The response from Delta was that the customer need not be at the gate to receive a complimentary upgrade, and that the GA acted out of policy/process. They would review with the local management team and work on additional training.