Mp4 Playback Issue in Published Course

Hi There,

I'm having a video playback issue in Captivate 2017 when published to HTML5. I have 7 videos (mp4) in a large course and the first two videos seem to get hung up in a perpetual "loading" stage in Internet Explorer 11. I'm not seeing that issue in any other browser, just IE 11.  We've only seen it happen in the first two videos which appear early in the course. Everything else plays beautifully. We have this happen in both a hosted version on GitHub and the client's LMS.

A few details on the videos. They are all imported into the project library, they are all set to Synchronized/Progressive download, encoded with H.264 and none of them are over 95 MB. In fact, the two that get hung up are both under 20mb.

Any ideas on how to approach this? Any advice is much appreciated!