What is the frequency of failed orders, how long has it been going on for?
There are worse things than negative SEO attacks, I have had to defend against two of my CC merchant accounts being targeted with malicious payments intended to destroy my client's business. You should talk with Stripe and sign up to their full Radar service for 1 month, have them fine tune the Radar for you as that is way to high of a failed payment ratio.
Another thing you may have not done yet is warm up the banks/cards to your account, meaning, find a wide variety of people/friends that use different banks/credit cards and have them call their bank ahead of time to approve their purchase on their site, or have them try to purchase and then call their bank immediatenly after to approve the transaction.
Once a few transactions are manually changed from each bank's anti-fraud to approved, future transactions with customers at that bank will allow no problem.