I am not sure I understand correctly.
The heading says daily digest not working. Do users who want a daily activity summary receive one? (They will only receive it when they do not visit the forum)
Are those from topics that they watch or are mentioned? Then the setting you are looking for is the “Email me when I am quoted, …”. If the emails notify about personal messages, it’s the one above. Both are set to send an email as long as you aren’t online. I think those who don’t want more than the activity summary want to set both to “never”.