Complaint Product Authenticity

I have an issue where a customer has made a product authencity complaint. I have submitted all the relevant invoices covering the current FBA stock and all sold items which confirms that the products I sell are authentic. Each time they are rejected and email received requesting further invoices. I have tried contacting the account health specialist through the relevant button on the Account Heath page with no response, just a silenced call when is immediately cut off. How do I speak to a person to resolve this issue. Any help would be much appreciated.