certain videos not playing in IE

I have a course which contains 13 short videos. Each of which is in a Lightbox. 

The videos were created in the same manner; encoded using the same settings and resulting in a standard 720HD Mp4 @30fps with an auto bit rate & audio at 48kHz stereo.

All but two of the videos play. They play perfectly in Safari, Chrome and Firefox - however our company's standard is IE 11(why?).

Two of the said 13 do not play. not automatically; not by clicking. They just don't play and I'm wondering if anyone has a solution to this conundrum please?

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