What is the player’s USB mode, under System settings, set to: MSC or MTP mode, or the dreaded Auto mode which can fluctuate between the 2? MTP mode can result in the situation mentioned by you. IMHO, MSC is the best mode–it just works, and doesn’t typically cause issues of this type.
If you’re not currently under MSC mode, you might try clearing the user content on the player (or transferring the content to your computer and then erasing it in the player), re-setting the player to MSC mode, and transferring your content back to the player, and then see if this works.
