This can happen for a few reasons:
1) You're not subscribed to the Mailsuite plan.
Individual tracking for multiple recipients is exclusive to the Mailsuite plan.
2) You sent the email before upgrading to the Mailsuite plan.
Mailtrack can't identify who opened multi-recipient emails sent before you upgraded to the Mailsuite plan. This is because the individual tracking wasn't enabled at the time of the send. So, Mailtrack won't know to track the recipients individually.
3) 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.
4) You've sent the email from Mobile Gmail App with add-on
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.
5) You sent the email to a Google Group
Individual tracking doesn't work for emails sent to Google Groups.