One reason is that people don't know the terminology given by PMI for PMB certification. They need to know the theology given by PMI, and they can find all the terminologies in one book.
A second reason is that people don't know the difference between tools and techniques, plans and documents. Knowing these differences is important for answering many questions on PMP certification.
A third reason is that people try to memorize ittos (inputs, tools, and techniques). This can be time-consuming and not necessary.
A fourth reason is that people don't practice enough. It's recommended to practice at least 800 questions in order to get a feel for the examination.
A fifth reason is getting stuck on one question during the exam. Spending too much time on one question can waste valuable time. It's advised to move on to the next question if you are stuck.
These are the reasons why people fail PMI. Wishing you all the best!
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