Completion report randomly incorrect in Moodle

Hi, I have a course which is tracked by slides viewed in a Moodle. Users should be able to pass the course by viewing 29/35. The problem I faced is that some users claimed that they have finished the course and even went for several times but won't get a completion, but actually some users have no problem to get completions for the same course. Are there any one who might have an idea what could cause this issue?

I am wondering if a user left the course, let say viewd 20 slides, and then came back the other day and finished the rest of slides (15 slides). Will moodle record the course completed since the user have viewed the total 35 slides in two attempts or it will report imcompleted because none of attemps reached 29 slides?

Ps: the course is set to prompt resume and published to SCORM1.2