Keep Checking - right up to departure. Put the seat map for the missing flight on your phone or computer or whatever and check it often. Many people who purchase F Seats often reschedule at the last minute.

Twice in the last week I've gotten free upgrades when the seat maps previously showed all seats taken, once just minutes before I got my seat.

On a recent trip on another airline I was unable to get seats together with my wife, until I was sitting in the gate for my departure. I changed the seats sitting in the terminal.