Inserting videos

I am using Articulate Studio '13. It is working fine but I am having trouble inserting videos into any particular slide. When I first insert a video (of any format such as MP4, etc.) it will work when I select "Preview this slide" but when I return to play it again, a blank screen appears. The blank screen has the correct "time" or "length" of the video appearing at the top of the screen, but no video appears. It plays as if a video should be there, but no image. I then reload or re-insert the video, and it will play, but when I leave the slide and return again, the problem persists---no video or image! Everything else in the slide presentation works fantastic.

Does anyone have any ideas on how to "retain" a video in a slide?

Is there a "best" or "certain" format that the video should be rendered in order to insure more stability?

Any help will be appreciated.

I have not attempted to publish a created Module or Course yet, but I am assuming that if the video isn't appearing during the preview phase then it won't if I publish it.

Thanks to anyone who can assist.

Bill