We all learn differently, and at a different pace.
Sometimes a student is perfectly capable of grasping a concept, but they are in a class that is too big, and they can’t get the attention they need from a teacher. Students may learn better visually, but the teacher uses an auditory teaching style. The student may learn best by doing, but the structure of the class demands that they sit in a seat for 30 or 45 minutes at a time, with no opportunity for experiential learning. Sometimes the pace of the class is too fast, and leaves the student behind. Other times the pace of the class is too slow, and the student tunes out. Despite a teacher’s best efforts to give each of his or her students what they need, it is virtually impossible to do so in a class of 25-30 students. Many teachers are forced to “teach to the middle”, and hope for the best.