Hi Namita,
you mentioned that "executions are not consistently increasing" - the system uses the Report subscription sender scheduled task for sending subscription emails, so if it's not being triggered, those emails are not sent. You can read more about this in our documentation.
I assume there's no consistent/regular traffic in your QA server. Kentico uses the Request-based scheduler mode for executing scheduled tasks. Therefore, there must be requests performed on your website in order to trigger scheduled tasks. You can change the scheduler mode to Automatic, as is described on the linked page, or you can set up execution of scheduled tasks by the Windows service. This will ensure that scheduled tasks won't depend on requests.