For a flight from DCA - ORD last week, I purchased an upgrade to first class and selected one of three seats remaining.

The day before, I had trouble checking in on the app and when I got to the airport, my boarding pass listed as first class, with no seat assignment.

I went to the gate agent and she told me there were no first class seats left and could not explain what happened. Now I have to try to get a refund etc. Very annoying experience.

Anyone know why that would happen and how I could have avoided it?

Thanks!