I failed too
I took the NAMAS course, studied a lot, passed all the practice tests in the book easily, and I failed too.
There are no good study materials for this test - you are referred to the CMS site, the CFR, etc etc, for more information. These all have voluminous books of information - they are NOT study guides. The mere fact that NAMAS points you in that direction demonstrates that they feel their study materials are inadequate.
My NAMAS instructor also said that they have not been allowed to look at the test, or a breakdown of the results of testing, so even they don't know have to improve their course.
AAPC does not sell any practice tests, so you can't tell what will be on the tests, or how things would be worded, or even how the test was organized.
This was really disappointing, and expensive. But I think the worse part of it is that I have no clue as to what to do to increase my skills so that I can pass a retake. I'm already an Auditor!