Time Machine Ventura - only allows manual backups

I have a an iMac backing up to an 18TB external drive. The TM backups include those from my previous iMac as well as backups from this machine since new. My large external media drive (plus Dropbox and Google Drive folders) are excluded from backups.

I started getting Disk Full errors and Time Machine would not complete automatic backups.

It used to be that TM deleted older backups to make new ones (although the backup drive is by no means full) as well as allowing selective opening and deletion of backups to files level.

I can continue to do manual backups (showing there is sufficient space).

I've read a number of reports of this but no simple solutions, other than wiping the drive and starting from scratch. I would prefer to keep my old machine backups (from old machine and also older backup history) although I might be able to transfer the old machine ones to another drive, as long as I can subsequently restore files from them if I want to).

Can anyone explain:

• what has actually happened to stop me making automatic backups (the drive obviously has space and is correctly formatted for TM to be able to make manual backups)?
• whether there is any fix for the issue?

Many thanks