[Kindle Oasis] Adding books problem

I have often added books formatted with Calibre to my Oasis kindle using transfer via USB from my computer. Have done this for years. However, last few days when I add a book to the kindle it transfers, I can see it in the kindle window on the computer, but cannot see it when disconnecting the kindle and looking for it. I can search but does not find though I know it is there. Have done with with half a dozen books. Have restarted the kindle twice. Still does not work.
Any idea?

–– Richard