Usually copy image and paste works for covers though occasionally not in my experience. You can always copy and paste in the description. If you have older books without descriptions/covers just highlight them, go to edit metadata and download covers and descriptions. This can be done in bulk if you want.
I have found that often when Calibre has not been able to find covers and/or metadata it's because there is something wrong with the title, author or isbn number in the metadata. Checking title author are accurate, and deleting the isbn number can help.