Calibre will only send one book to kobo

Hi,
I am a complete newbie. Everytime I try to add books to my kobo, calibre will only send one. If I try to send another, it will delete the first one.
I messed with my settings ages ago to try and make it so I had bookshelfs based on the column 'genre', but this only happened when I updated my kobo. I thought maybe it would get fixed when Calibre updated, but it didn't.

I know nothing about personalizing calibre and every time I've done anything I had to follow a tutorial, but now I can't download books and I have no idea how to fix it!

Please help!