If you have failed the test and have looked to the score sheet that they give you after you finish the exam and still do not understand why you have failed, here is why: Zend does not accept any score lower than Intermediate (medium) in an exam and if you got that, you have failed.
You have for the exam 12 categories, arrays, web features, xml, design, differences between PHP4 and 5, etc. All categories are marked with low, medium and high scores, or better, knowledge level. To pass the exam all you need to do is have a knowledge level of medium / high in all categories to pass.
I got this information after talking with the Zend customer support (and off course, I have asked about the confidentiality policy to know if I could disclose that) and hey, I got some good news, if you have taken the exam before, you are eligible for getting an discount to retake the exam. All you got do is sending an email to firstname.lastname@example.org.
Good luck on your next exam.