1) I am a frequent flier, with over 420,000 miles STILL in my Delta account.
2) Having not enough open seats depends upon how early you book.
3) The closer to the flight, the fewer seats truly open.
4) Frequernt fliers are upgraded at 5-day, 3-day, 24-hr pre-flight periods.
5) Inability to get a seat assignment is due to one of these
. . . a) flight is already overbooked
. . . b) frequent fliers in Coach are not yet upgraded (5-3-1 day free upgrades)
. . . c) there is a tour that had pre-booked seats not yet released
NOTE1: Prior to the 5-day advance date, in most cases, it is 2b. But,
if less than 5-days prior to flight date, it is probably due to 2a and you
may be in big trouble and likely to be bumped. Best advice is to check
several times a day to see if seats opened. If not, get to the airport
at least 3-hrs ahead of time. The earliest people at the counter or the
gate are the ones who are first ones to get on the wait-list and the first
ones to get any last-minute available seats. (Airlines can overbook by 10%
per law. So, when I see a flight without open seats, I pick another flight.
Just because they sold you a ticket, it does not guarantee you a seat on
that plane)
NOTE2: If the seats are not available because they are being saved for
frequent fliers, then the seats are simply "gray" versus being noted as
unavailable. This means that the seats are not bought and could be
assigned later. If the seat is "unavailable", then it is sold and the flight
is probably overbooked.