Some mail clients do not allow for open tracking.
Some users may have marked it as spam and their service may have unsubscribed via the headers.
Some blacklisting may appear in the unsubscribes (and some of the blacklisting rules can be adjusted in mwz).
Have you tried testing your open tracking in various clients?
in the file
you can make custom rules based on the original (keep it as is) rules.php
(simply copy the original into your custom file and make adjustments there).