Time Machine Backup - no subsequent backups

I have completed initial Time Machine backup, however subsequent backups, even after several days, do not even start. I am a 20 yr plus, savvy Mac user. I can still backup to Time Capsule, but no subsequent backups to the WD NAS.
Appreciate any suggestions.

Hi @vinnib Please refer to this KB article : Starting macOS Time Machine Backups to My Cloud and Network Attached Storage