Content freezing in LMS - Some colleagues are still able to complete

I developed a organisation wide eLearning project which has now been published to Cornerstone LMS. Some colleagues reported that were unable to complete due to the content freezing (all at different parts of module). The same colleagues managed to complete after a few attempts - we put this down to wi-fi connection.

My colleague and I have investigated this on both mobile and desktop, and using different platforms including safari - all worked fine from the LMS.

As this is happening quite frequently, I was wondering if I could send the file over to you to try and spot any errors that my colleague and I cannot see; alternatively, any other suggestions?