Time Machine Full

I have a MacBook Pro with the latest Sequoia installed and 2TB of storage and a 2TB exterior drive that's my Time Machine source. During my latest backup, I received a message that my Time Machine drive was full and the backup couldn't be completed. Time Machine's info indicates that when its drive is full, it will delete the oldest backups to allow a current backup to take place. I'd appreciate help with why the auto deletion to allow a full backup is not taking place (I have just under 700GB of data on the Mac, so space shouldn't be a problem. Thanks!