Some reasons why a user would not receive an email from an Email or File Share action:
The action was a File Share action for which the Email Notification property was set to No.
The user for whom the action was processed has a disabled user profile status in Stratum.Viewer.
The processed action resulted in a “no data exists” case and the Send Email when no Data Exists property for the action was set to No. See also What Does “No Data Exists” Mean?.
The user’s email client may have treated the email as spam and routed the email to a junk or spam email folder. Emails can be treated as spam for a variety of reasons depending on your mail client, such as because something was considered unsafe in the subject line or body text or because the sender was not considered a safe sender by the client.