"On Device" not working after Calibre restart?

Hi there :)

I've just got a new e-reader, a Kobo Clara BW, which I'm enjoying a lot.
I've installed Koreader (recent nightly) and am using Calibre to sync books using USB.

Calibre sees my device, I can send books to main memory and it will show a green checkmark in "On Device". However, if I restart Calibre or disconnect and reconnect, the "On Device" status is cleared.

I see no option to scan the e-reader for books and if I go to Device -> Show books in the main memory of the device, no books show up.

I've tried searching these forums and elsewhere (reddit, google), but I've not found a solution yet.

Does anyone have a suggestion / tip? Thank you.