Barriers to passing
While every student in a class receives the same lesson, all students do not fully understand or retain all the information. Some of this is related to the students' intelligence and attention level, but much of it is related to how dedicated the students are to learning and how much effort they are willing to put into learning. Students who persevere the most and work the hardest, learn the most.
Not every student is destined to become an instructor. Since Taekwondo is a physical art, all students must also be able to perform physically what they have learned. However, while many students are skilled at physical techniques and understand all the principles of Taekwondo, they are unable to articulate them. The ability to take instruction and the ability to give it are two distinct competencies.
Language may also hamper the passing of information. A person may be a highly competent instructor in his or her own native language, but, when instructing in another language, the instructor's competency in that language will determine the quality of the instruction and the amount of information that is passed.