Need some more background, Do you have status with Delta, if you don't then there is a batch of seats that you will not be able to book as they are reserved to 24 hrs before departure. On CRJ without FC usually 1a and D are blocked off till the last minute for weight issue or passengers with speical needs. As for the musical chairs it's more then likely a combination of people being upgraded to FC and people doing exactly like you are trying to score a better seat.