Hi Clint!
Since you have a daily PM-Import execution, anyway you have a latest updates imported which released date (What was a reason that mails weren't arrive to mailbox from NS, I don't know, there could be different reasons, like SMTP server unavailable, Task Server queue was exceeded by Server side tasks execution so send mail task wasn't able to be successfully executed in exact date/time on NS, etc..)
Just a quick overview about "New Software Bulletin Available Summary" automatin policy:
This Automation policy sql query gets list of new bulletins which released date is < 24 hours than today's date on NS. You can edit this Query and set another condition, otherwise change current query parameter 'TimeUnit' from hh to dd and change 'TimeValue' from 24 to required value.