It's a simple and complex issue. In short, the video encoding is not one the KF HD 7 can decode.
Fix? Try another encoding or research what encoding this tablet supports then encode it to what it can use.
<span style="font-family: verdana, arial, helvetica, sans-serif; font-size: 12px; line-height: 16px;">Hey guys. I got my new Kindle Fire HD for Christmas. And I transferred a personal video from my computer to my Kindle via USB. It's in MP4 video format and it's about a few hours long. But, when I go to try to play it, a message pops up saying the following: "Unable to Play Video - Sorry, this video cannot be played." Why is this happening? Thanks!