So here are some answers why you would not receive an email...
Questions and Answers:
Q: How often are emails sent?
A: Every 10 minutes.
Q: If the member looks at the Alerts notification before getting the email, will the email still be sent?
A: No.
Q: Does this add-on send an email if the member has a state other than valid?
A: No.
Q: Does this add-on send an email if the member has "Receive site mailings" unchecked?
A: No.
Q: Does this add-on send an email if the member has been banned?
A: No.
Q: Does this add-on send an email if the post is being edited and a user tag is added?
A: No. Only new posts with user tags get an alert.
Q: Does this add-on send an email if the member has been inactive for a long time?
A: No. If a user has been inactive for over 6 months they will not get an alert.
Q: Are the user group permissions that can prevent an alert?
A: Yes. The Maximum mention alerts can prevent an alert from being sent.