I was #2 on the UL with 1 seat open up front when boarding started. The flight boarded and first class was full. Oh well, better luck next time.

The flight then proceeds to get delayed multiple times due to "congestion" at the airport. Eventually, 2 hours after the boarding door was first closed, passengers were given the chance to leave the plane with the understanding that they would not be able to reboard (and their seat would be surrendered) if and when the flight actually left. I stayed on the plane.

2 passengers from up front decide to leave at this point along with several from Y leaving 2 open seats in F. I asked the FA if it would be possible to move up front at that point, or check with the GA if my upgrade could clear, given that the seats were open and my position on the list was such that I probably would have received the upgrade. FA insisted that I'd have to go to the gate in person to ask that question, which would have been fine except had I left the plane, I would not have been allowed back on. She further insisted they could not radio to the gate to ask this question.
(We then got stuck on the tarmac for another hour and a half before taking off, with 2 open seats in F). The service was fine and the crew made the best of an otherwise nasty situation, but of course it would have been nicer to be sitting up front.

My question is whether I a) have any grounds for a complaint to Delta based on this, and b) whether the FA was just brushing me off or that the FA's actually cannot do what I was asking.

I am genuinely confused by the whole situation so any advice is appreciated. Thank you!