In case of @natasha, pretty sure it's coz of the i821 that they said the amount was incorect. The guys who process don't use their head and are like chakko pilo, just makes your life difficult. They don't bother thinking twice. Just send i765 and not the i821 this time around.
In case of @chabahil, they require a copy of your TPS approval, which in itself is a i797, from what I understand. If you've for that you just need to send the latest i797 for the i821 and you should be good.
@pyackman: have you received your i797 yet, or any email sms notification where they provide you with the case tracking number?