I suggest you try 'sideloading' a couple of books to the PW - i.e. via Windows Explorer. (Maybe you already did this)
If that fails then try sideloading on a different PC
- if that fails then the PW or its cable is most probably 'the problem',
- if that works then your PC is most probably 'the problem'.
If that (sideloading on your PC) works then maybe its the books you're trying to transfer. Convert a plain text file to AZW3 with calibre and try sending that to PW
- if that doesn't work then we're back to square one

- otherwise 'the problem' would seem to be with the books you're trying to send
I don't know what the ramifications of resetting the PW back to factory settings would be. I imagine it would be a lot less than resetting your PC back to its factory settings.
BR