First, call your FSA provider customer service to ask why the claim was denied.
If your claim was denied to due to the confirmation receipt, check to make sure the receipt as everything you need on it: Provider Name, Date of Service, Product Name and Amount
Date of Service has to be a date. If it is a timestamp (i.e Today at 10:27 AM), just wait until it turns into a date. Some email clients use a timestamp for the first 24 hours.