Time Machine Malfunction

[scottsan]

I'm running high sierra on an iMac and use a 2 TB external usb disk for time machine backups. Time machine is not automatically deleting old backups to make room for new ones. Instead I get a message that the backup cannot be completed because there is not enough free space. Further, if I enter Time Machine, navigate to an old backup to delete it manually, select a backup, and then provide admin password and confirm that I want to delete all backups for that day, all backups are deleted, not just for the one day I selected.

Any ideas out there?

Thanks in advance.