My Book 8TB - Will not reliably go to sleep/power down

So I’ve downloaded the latest WD Drive Utilities (2.0.25), and used it to set a time (eg: 15mins). Tried setting a PowerSaving drive powerdown in Windows 10 to various times (including 0 to disable it), and more often than not the drive is simply just staying up, even long after the PC has powered down the monitor due to no use for 15 mins…

It has sometimes powered down, but that seems the exception to the rule!

Am I missing a trick here? I don’t want the drive after it’s not been used for 15mins! Else it could be on for tens of hours not used!