Have you tried other video formats, e.g., .avi, .flv, etc? My .mp4 movie file plays fine on the extSdCard with the stock Video Player and QQ Player and ES Media Player (from ES File Manager). Have you tried playing the file on your PC or otherwise ascertained that the files are playable?
Actually, if it's playable on the tablet but not on SD, it's probably not a player issue but something else.