As someone who tutored two people who failed boards from my residency, take this for what it's worth but I think the majority of people who fail this exam don't know the basics. The exam is not hard. What trips people up is diagnostic criteria and things they may not have seen. Every psychiatry resident (hopefully) knows first line and second line and third line treatments for depression. Where people tend to have trouble is the more academic questions like distinguishing between the different types of depression, how many criteria is needed to make the diagnosis, what the difference is treatment is and why. This is all stuff from the DSM. Know it. Well. Like the back of your hand. There are going to be ridiculous questions like how many ADHD criteria do you need if the patient is being diagnosed over the age of 17. Or that you shouldn't make the diagnosis of DMDD before age 6 or after age 18. Or that DMDD symptoms have to be present for at least 12 months and if the patient went 3 mths in a row without all the symptoms, they don't meet criteria. All these nuances may be of little utility in real life, but it can be the difference between a pass and a fail if you miss enough of them. The other thing is epidemiology. How many people have schizophrenia? Is it male or female predominant? All these are in the DSM as well.
There are other reasons people fail but this is a major one. When I first started tutoring, I didn't realize that people get through residency without knowing the DSM. I learned the DSM first thing, before I ever picked up a peer review article. But I realized really fast that the ones who failed didn't know these little annoying things that won't make them a bad psychiatrist but will make them fail the exam.
LEARN THE DSM FIRST! All the other study materials are secondary.