Time Machine not backing up automatically

I have a Western Digital external drive (formatted as APFS, encrypted) connected to my MacBook Air, which runs macOS Ventura 13.3.1. The password to the drive is stored in Keychain. I am expecting Time Machine to perform hourly backups, but it's not doing any automatic backups. The MacBook Air is connected to electricity (not running on battery)

If I open Time Machine and click on Back Up Now, the backup runs fine. Any thoughts on how get Time Machine to back up automatically?

FYI that I also have a MacBook Pro, running the same macOS, and connected to the same model of Western Digital external drive (also formatted APFS, encrypted). On the MBP, Time Machine auto backups work fine. So am puzzled why it's working ok one Mac but not on the other.

Thx.