FEMA asks for missing information through a request for information (RFI) with a deadline of at least 15 days. Miss it without an approved extension and FEMA denies whatever it couldn’t verify. Any denial comes as a determination memo explaining what was denied, why, and how to appeal.
Your complete RFI response, any extension request and approval, and the determination memo with its transmittal date.
Treat every RFI response as the first draft of your appeal. FEMA decides appeals on the administrative record, and the RFI response is usually its strongest piece.