The most common reason this happens is using a corporate/business card, but using the incorrect name or address. Sometimes the business name needs to be used, but other times the individual user's name needs to be used; depends on how the credit card was set up. Also, sometimes a corporate/business card will have the business's address on file, and other times it will be the individual user's home address; depends on how the credit card was set up.
Also, we sometimes see people not entering their Verified by Visa password or MasterCard SecureCode password, which are security features some people have on their credit cards. The feature is something that the card user can enable/disable on their credit card, or it can be turned on by default by the credit card company. If you have Verified by Visa or MasterCard SecureCode, and do not enter the password on the AlertFirst payment screen, your transaction will be declined by the credit card company.