Video MP4 won't play in course

I have built a course for a client and when I uploaded the web output file to our company server, the link works just fine - all sections play correctly. I have sent the same output folder to our client and had them upload the same content to their server. They have pointed a link on their to the "story.html" file. When I view their content, many of the mp4 files are missing from sections within the course. They have verified all 115 mp4 are uploaded, but some of the content will not show on their uploaded course. Any advice on why these videos will not play? 

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