MP4 video will not show in Preview or publish of Storyline course

Hello Kerry, 

Since you mentioned that another MP4 file worked without issue, this most likely means that the behavior may be occurring because of how your videos are being encoded. If you have access to a video editing tool, try re-encoding the problematic videos as what my colleague Maria suggested to see if this helps. You can also reach out to our support team by opening a case so we can take a look at your videos to see what's going on. NDAs can be provided if this will help you share your file with us.