Failed in the registration exam

I plan to register for the OCA(Implementation Foundations) exam. However, after I entered my credit card information, according to the registration guide, the page was supposed to be redirected to the MonitorEDU interface and then I could choose the exam time. But my interface didn't jump over, instead, it returned to the registration exam interface. I tried several times, and it was possible to register for other exams(for Example OCA R2) and  succeeded, but only for the OCA exam, the registration failed. Does anyone know the reason?