Hi,
Based on your description, my understanding is that sometimes the message trace report doesn't record the time when the email was viewed and you have an Exchange hybrid environment. Please point out if I am wrong in any way.
I did some testing on this, and eventually realized that: when the email is sent successfully, the recipient gets a warning, like in the picture below :
and if the recipient selects No or in fact doesn't open the email,, you won't see the Read item data in the Tracking Report, like this:
Hope this helps!