The failure notice may sometimes report what the problem was, but tit doesn't have to. It may simply state that they were unable to determine the authenticity of the item. The service is provided to eBay, not the seller, and eBay won't get into a debate about the authenticator's opinion.
My advice would be to check the item description carefully in case anything could have been taken as misdescribing it, also that it doesn't contravene any of eBay's listing policies.
If you want to verify the authenticity of an item you can usually get advice from an authorised distributor.