This article gives more explanations on how "Monitor Activity" works
Please keep in mind, that “Monitor Activity” can take some minutes to refresh available data. For performance reasons, message processing and message monitoring are disconnected. This means that it can take up to 10 minutes in the worst case until you can see your message.
Another reason can be that the credentials used for sending the message were not correct, in that case the message is rejected and it’s not tracked in the monitor activity.