It's the video encoding.
by R. Proffitt - 12/26/12 7:52 AM
In Reply to: Kindle Fire HD 7": Troubles with Playing Personal Video by valhom
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.