If a customer has an issue with their payment method, they'll be presented with a generic error message. The reason they will not receive a detailed error message (card invalid, address mismatch, etc) is that credit card scammers utilize such data to test stolen credit card numbers. However, we do save this data so that you, as the store admin, can access it to help a customer out. We log all credit card gateway data and make it available under Payment -> Gateway Log in the seller admin.
Also note that multiple failed attempts to complete a transaction in a very short time will result in the buyer being auto-blocked in Cartloom. This is again to protect you from fraudulent activity or your Cartloom account getting abused by scammers to test card numbers. Since many payment gateways charge 10-25 cents per authorization, we actively prevent a malicious user attempting to test thousands of card numbers (at 25 cents each!) with your Cartloom account. If a user has had issues submitting payment, check and make sure that they're not on the Block List - you can view/remove blocks under the Tools -> Order Blocking menu.