Following up on this. KCC actually sent me an email yesterday mentioning the particular document that was missing. So, I think you were wrong to say "KCC does not write to ask for specific documents if they are missing". If that used to be the case, then be informed that it has changed.
The issue was with my passport. I entered with a different passport which expired later and I filled DS260 with a new passport. But upon submitting my documents, I had scanned both passports together and attached them as one file. Apparently, they wanted me to explain the reason for the passport change, which to me seems a little weird because the passports are clearly attached with the expiry dates of both, seen. But well, rules are rules.
So, now they asked me to resend with an explanation. That's the update.
Where I need help now is to understand how the explanation is done and submitted.
Appreciate anyone who's done this before.
Regards,