It's important to understand that information about opens are based on us having a direct proof that such action has been performed by the recipient. If you see that a specific recipient you are interested in didn't open the email it doesn't necessarily mean he didn't - it only means that we did not receive the proof of opening. The proof is evaluated based on an invisible tracking image included with every email that is sent. The moment the system records that the image has been displayed on the recipients screen it gets marked as opened. In some cases (email client setting, device used by the recipient, etc.) this tracking image can not be displayed and therefor we don't get the proof of open.
Using these hidden tracking images is the only accurate way of measuring email open and if anyone tells you otherwise he's very likely lying to you. (But if you happen to know someone who found another - more accurate method to measure opens - please send us his/her contact so we can get in touch and offer him/her a job)