subscription expiration due mail

-- HikaShop version -- : 4.4.0
-- HikaSerial version -- : 3.1.2
-- Joomla version -- : 3.9.22
-- PHP version -- : 7.3

Hi,

We have a query on subscription expiration due notice cron setup.
We set the "Minimum delay between two cron tasks" as 1 minute, and the "frequency" value is in 15 minutes. We set the days before as "1 day" for subscription due emails.
But we are unsure about the difference between Minimum delay between two cron tasks & frequency values. Can you please share in detail?
Also, sometimes we receive expiration notice email as expected and sometimes not. Though we change the "subscription end date" manually, emails are not received.

Thanks,