movies in general

hi we got a lot of movies over the years and decide to put on to a hard drive , so what i have done was to reconvert from disc to mp4 useing wondershare and then put onto hard drive and they worked fine , but when i download a movie that is either mkv , avi , mp4 and convert it to mp4 put it on hard drive it dont work and yet i am converting the same way so why is it that some movies will work and yet others do not , i am so confused why it is not working are mp4 and h264 the same file we have now tried a different convertor Xilisoft Video Converter Ultimate and the same thing happens it is not seen on the hard drive i have tried avi . mkv . mpeg 1 and 2 . divx. h264 etc and still cannot see the movie on the hard drive and yet all other movies are converted to mp4 h264 and work fine so why is this happening even some movies that have been on disc and been converted work and some dont but converted the same way .

thank you for your reply i have now got my money back from this company as they could not help me to solve this issue