Geeze people, stop GUESSING at this and GET SOME REAL FACTS BY ASK YOUR CUSTOMERS!
Post your phone number on your website and invite customers to call you if there are any problems. You won't get crank calls. Instead, you'll get good calls from honest people who's cards are being declined. Get their names and email address and then call your processing company and ask them why this customer is being declined.
We have done this for a while now and the reasons range from IP / Geo Mismatch, or entire domains blocked for fraud. If you know your customes, you may know that Mr. Smith is a travelling salesman, or that Mr. Jones is a repeat customer, and that neither of them should be declined for these reasons.
Next, ask your processor to remove the blocks on these customers. Call your customer right back and invite him to try again, and voila! Not only have you gained a sale, but if you word it the right way, you (and your processor) were simply "looking out for your customer's best interest and protecting him from identity theft".
We learned this years ago as a part of our own internal fraud scrubbing (see
http://2much.net/fraud-detection.php) and it works very well. While it's against CCBill's TOS to DEMAND that customers call only you, I am sure they appreciate it if you go out of your way to assist with customer service and do some of the leg work for them. Get the info from your customers, square it away with CCBill (or whoever), and watch your declines go down.
