Moving movies from laptop via sub problems

[FeelingTookish]

So I got a Kindle Fire HD and I have downloaded a movie and I converted it from avi to mp4 as per usual.

Although every time I have copied it to my 'movies' folder in my laptop and it copied no problem (although it does say it'll be down in about 6hours and it take ages) but every time I go to personal videos the thumbnail is blank and once I click it, it says ' Unable to play video'

I don't know what to do please help!
Thanks
~Emmy