Check to see if the USB mode is set to Auto Detect. In this setting, it can connect in either MTP or MSC mode at will (and without your knowledge). While the player will still see and play files loaded in either mode, your computer can only see the files that were added in the mode it is currently connected in, not the other.
Set to either MSC or MTP and then connect. Are the “missing” files visible now? No? Then disconnect, switch to the other mode and try again.
For a long-term (and less confusing) solution, delete all the files (again, you have to do this is the mode they were originally loaded in); although best way to do this (without changing modes) is the format the player. Then pick 1 mode (MTP or MSC) and set your player to it, and then re-load everything back again.
Which one? Depends on your usage. Do you listen to DRM-encrypted files (Audible, protected .wma files, etc.)? Then use MTP mode. If not, and you own all the music, etc. use MSC mode.
For more info on these 2 modes, use the Search function here; there are literally hundreds (if not more) posts/threads on the subject especially with these older players that have these 2 USB mode choices. It is not advised to use the Auto setting.
