There can be many reasons. I am sorry but I am on linux and may not be of much help right now.
Can you please check this
Does any of the answer applies to your case?
In that case, I am sorry but you will have to wait till we release the FFMPEG encoding addon. That will do it automatically(convert into formats).
Also, Can you please tell me if the same video works correctly or not if you do it via WordPress media uploader?
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