usb extrenal drive files not listed

Hi,

I habe a panasonic bluray player DBT-110, and I have a problem when I coonect an external usb HD. First of all, I format the drive to FAT32, in order to be recognised. Then I copy movie files from my win xp laptop to the drive.
Then I coonect the drive to the player, and I can see the files listed in the player explorer.
After that, I add more files to the drive from my laptop. I can see all them in the windows explorer.Then, when I connect again the drive to the player, some files will not be listed anymore.
The same happens to folders, or other files. Whenever I add files to the HD, it can happen than other files won't be listed anymore in the player explorer, although I know they are there.
Anybody knows what happens?

Thanks