mp4 video not playing when published

I am revising a module that was created in Storyline 2.  I'm working in Storyline 3.  I've reinserted the video because it wasn't working, but it's still not working.  (The video is on a layer). If I hit play on the timeline, the video plays,  but if I preview or publish the slide/module when it launches the layer with the video, I just get a black screen (with a VERY slowly moving progress bar).

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