In case if your emails are not getting tracked, here are the conditions:

You have setup a custom tracking domain but you are not using the unsubscribe URL at the time of sending out the emails.
You have setup your custom tracking domain incorrectly.
The user is using an extension where he can block any emails being tracked.

How to resolve the problem? 

Either use Unsubscribe URL if you have setup custom tracking domain
Remove custom tracking domain completely and do not use unsubscribe link while sending out the emails.
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