Dear Nachha,
I would recommend you check
1.If the mode of the user’s enrollment is correct. You can check it by going to the course enrollments table on the Django admin panel or by downloading a users report from the instructor panel.
2. Whether the grading policy of the course is correctly set
3. If the option to auto generate certificates for students is enabled. You can enable it from the instructor tab-certificates if the course is instructor-paced. If the course is self-pace, you can go to the Django admin panel /admin/certificates/certificategenerationcoursesetting/ and enable it.
4. Whether the course mode is set correctly(this might sound obvious but double check does not make any harm 
)
5. If the certificates are configured to be delivered after the student passes the course. You can go to studio-settings-advanced settings- Certificates Display Behavior.
“end”, allow users getting the certificates once the course is finished
“early_no_info.” allows users getting the certificates as soon as they pass the course according to the grading policy
I hope this helps.