play MP4 file under 'File'

hi Guys,
i've got a 16GB 3G IPAD purchased recently (no jailbreak was done due to 3.2.2), i purchased a programme calls "File HD" (a document reader for IPAD ) from apple store and copied some *.mp4 movies into it via WIFI. it played all good for the initial few days, but now every time i press the play button for these MP4 files, it just goes freeze and wont move at all.

i've also sync same file into default 'vedio' path (dont even know what apple call these delault stuff as folders or what?? i mean the normal standard photo, vedio ipad directoy you got when purchased) , where all files played good.

the same thing happens if you use downloads manager to download *.mp4 and when those downloaded files are stored in their specified folder, the movie just wont play. (the file will open but the movie just wont move).

just wonder if anyone's got same problem as i do? anyway to fix it up?

thanks a lot.

regards