Thank you both! We discussed it again with the team and did some tests, and we figured that it might be because the team always used the same email in Marketo, just updated it with the new content. I believe opens won't be counted in case a person opened the email before (any of its previous versions I mean). So most probably we created the problem ourselves But I will ask our developers to review the code one more time, just to be 100% sure. Thanks!
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