The video has to be compatible with Silverlight. Is the video a WMV? Also, your Silverlight currently needs to be the same protocol as your video file (e.g. https and https, or http and http. can't mix http with https).
The documentation discusses what is supported as far as Media (see MediaElement). I think that its WMV, WMA and MP3 but I dont' have it in front of me.
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