Seat assignments are subject to change for many reasons and seat requests are requests that are not guarantees. Also, if you purchase tickets thru non airline websites, they send your requests to the airlines and if the seats are available they will assign them to you, but if they are not available they either dont assign seats and you get whatever there is when you check in or they assign whatever comes back as available. Third party websites might not be updated as often as airline sites so the seats available might not be as accurate and also any purchase seats are not able to be sold via these websites. Ive often seen customers say they selected these seats and had them on their printout, but they should never have shown as available to choose to begin with. Sometimes it might appear that an aircraft change hasnt occurred as you state, but depending on how far out you bought your ticket this might not be the case. Say you bought your ticket in January for a flight in October and select seats. The airline shows a 737 on this flight and thats the seatmap. Now in Feb they upload a schedule change and show an MD80 on this flight which would change eveyones seats possibly because you lose a middle seat down one side of the plane and add a lot of rows in the back. Now theres another schedule change in June and it goes to a 757 which means its back to 3-3 seating, but the exit rows and preferred seats would be different and now a lot of seats have changes again. Then in Aug, they go back again to a 737 for the Oct flight, the flight you thought you had booked originally. Unless you've been tracking the flight you would never know that they've changed aircraft types 3 times since you booked and changed your seats each time. You can call the airline now to see what they show for your seats. Of course, they're still subject to change, but if they arent together and cant be together now you know that you need to online checkin 24 hours before your flight to try to change seats if anything opens up.