1) You're not subscribed to Mailtrack Advanced.
2) You sent the email before upgrading to Mailtrack Advanced.
3) You're scheduling emails to send later
Currently, the individual tracking won't work with emails that have been scheduled to be sent at a later time/date. We're working on it, though!
4) You've sent the email to more than 50 people
The individual tracking doesn't work with emails sent to more than 50 people. We're looking to change that but in the meantime, if you need to send a mail to a large number of people we recommend using Mailtrack Campaigns.
5) You've sent the email from your phone
The Mailtrack mobile add-on has limited features and doesn't have the ability to individually track emails sent to multiple recipients. This FAQ explains the difference between the mobile add-on and the browser extension.