Quizmaker randomly not sending results to Moodle

Moodle 9.1.12+ with all patches applied. Quizmaker '09 scorm 1.2 content set to report score and passed/failed status based on user's score. 75% to pass the quiz. Submit all at once option enabled in Quizmaker. Users are using IE7 and IE8 with Flash player 7 and above. This is working fine for most of users. Score and quiz status are beeing recorded to Moodle's database and showing in gradebook. the problem is that some users are reporting that their score is not recorded although they passed the quiz and Quizmaker's results window was displayed to them saying that they passed the quiz. Moodle's database contains few tracking records for those users, for example:

cmi.core.lesson_status=incomplete
cmi.core.exit=suspend
cmo_core.total_time=00:20:11.46
x.start.time=1320855737

There's no score or interactions recorded.

It is extremely difficult to track down because I can't reproduce this behavior and it happens randomly. Has anyone encountered similar problem and how was it solved?