Cheqbook requires captchas after the first failed attempt, and will lock your account after a fourth failed credit card attempt due to "Card Testing".
Card testing is how credit card thieves test card numbers they've stolen. They try to run small charges online, especially at credit card accepting sites they can program to automatically test with, like Cheqbook. If a card fails, they know it's bad, if it works they can use that card number for larger frauds.
The captcha we require prevents them from using Cheqbook to automate this task, and the account locking prevents them from continuing manually.
We warn you after the third failed attempt that your account will be locked. You should not continue trying at that point, at the very least you should verify your credit card is valid with your bank beforehand and tell them this charge will be coming through from Cheqbook.
If for some reason you're just having a bad day and triggered this security system in error, we can unlock it for you. Please reach out to email@example.com and share your phone number. For security purposes we will have to personally verify some information with you before we can unlock the account.