First, students have not completed enough studies within the specified time, or to put it another way, during the four-year study period, a large number of students failed to make-up exams, and the state stipulates that schools cannot issue diplomas to students.
Second, students who have serious violations of discipline during college, such as fighting, gambling, crime, cheating, etc., cannot be given graduation certificates, and may be ordered to drop out of school in serious cases.
Third, students failed to pay tuition fees within the specified time. If they delay their tuition, they need to pay it before graduation. If they don't pay the tuition, according to the regulations, they won't get the diploma.
Fourth, other reasons. Some schools clearly stipulate that you must pass CET-4 to get a diploma. You need to ask different schools about this. Some schools don't. You can get a diploma directly after passing Band 4.