Cannot delete and copy files from MP4 player
 

Brand is DV MP4 MP3 player 1GB.
I seldom use it. yesterday i copied 6 videos into it. and now i cant delete
or copy new files into it.I try to delete it from MP4, it will show delete but later i
scroll out and scroll in again, the files is still there.
I also tried with my PC and it the same either. I also tried Window Explorer and also same.
Now i tried to copy new files in it, it will show succesful but when i disconnect it from my pc, and open the MP4, i cant find the files.
Only these 6 videos that i have trouble deleting.
I also tried error checking and format FAT via PC and it say 'unsuccesful'