Hi Martin,
The generic answers to your generic question is that questions like that are best answered by your ARTCC training staff. It also depends on which exam you failed. All that said, a safe answer would be to take the time in between the failed attempt and the next time you are eligible to study the material a bit. Also, keep in mind, all VATUSA written exams are open book -- you should reference the VATUSA training material and/or the FAA Order 7110.65 and/or the FAR/AIM when you are taking your exams.
EDIT: I just looked up your record. It was the VATUSA Basic ATC exam. You have a 7 day waiting period, so are eligible to attempt a retake on 9/8. I'd suggest you take a look at the questions you scored incorrectly and research WHY you answered the questions incorrectly. Just like any exam, we don't care (really) if you answer an exam question correctly or not, we care if you understand the material, or know where to look it up so that you can understand the material. The good news is that for you, there's one stop shopping -- alll of the answers to the questions you answered incorrectly can be found on the VATUSA training site.