Can you please copy/paste the error logs?

When you get the loading icon, press F12 (or Ctrl+Shift+I), then click the Console tab. There you should have error messages describing the error occurred.

Even a screenshot of the console might be useful.

Then let me suggest you to look for an automated testing solution, where your courses are tested simultaneously over many browsers and versions across multiple OS.

Have look to this service: http://browserstak.com

They talk about testing web sites, but actually it applies to any HTML5 content, including adapt courses.

Using such kind of tool is a good way to drastically reduce the risk of having learners getting problems on a course. Bugs happen even to best class software and courses.

If you are interested, I developed testing with similar technologies.

Cheers, Fabio.