To check what is happening:
- You send the book to the device. Calibre should show the book in the device list.
- When you look at the drive for the device, the book is there as a kepub.
- When you eject the device, the book is imported by the device.
- When you connect again, calibre is not showing the book in the device list but you can still see the book through Explorer or whatever you use.
Is that it? If so, the books should be in the device list. If it isn't, hopefully a debug log would show it.
If it is something else, I need details. For example, if you restart calibre before ejecting the device, the book will not be in the device list. This is because the driver reads the database and any books not in the database are not found. Calibre knows about them until you close calibre, but after that, they are not seen until the device imports them.