This is a pure guess, but maybe a user has muted a category, topic or tag (and maybe there is no log of that reason for not emailing).
I had a similar experience but never looked into it in detail in the end:
This is a pure guess, but maybe a user has muted a category, topic or tag (and maybe there is no log of that reason for not emailing).
I had a similar experience but never looked into it in detail in the end: