Issue with multi-module SCORM course

Hello all! 

I've been running into an issue with a Storyline 1 course and was hoping for some assistance.

We offer a lengthy (4+ hour, 6 module) course on our LMS. Each module is set up as a separate scene.  In order to move through the course, learners must unlock the next module by completing the previous module's quiz. I have a True/False variable set up on each module. The modules are unlocked on a landing page that the learner is brought to after completing each module. 

Some learners are reporting that they complete several modules, and then when they return to their training they’re brought back to Module 1 (even after clicking “Yes” to resume where they left off).

Has anyone experienced this? Any idea as to why this may be happening? Just trying to figure out if it's something I can solve in the source file, or if it's likely an external problem relating to the LMS or the browser they are using. 

I should mentioned that this issue is only happening to a small number of people taking the training - likely less than 5%. Which is why identifying an issue has been so tricky!

Please let me know if you need me to provide any more info. 

Thanks in advance for your help!