We are using Journey Builder to send out transactional emails. I am reviewing one of these journeys and I can see that the email activity has a small number of "Not Sent", which I assume means they entered the journey and marketing cloud decided not to send to them for some reason.
The email activity uses a Transactional send classification (since it is a transactional email), which as I understand ignores subscription status, so I'm guessing it's not that.
Is there a way that I can find out why they were not sent? I couldn't find anything in the documentation about what this actually means.
 just wanted to add that while it is possible to click on some of the numbers and percentages to drill down, the Not Sent number is not clickable.