Audible downloads, in iTunes/Audiobooks, doesn't sync to iPod?

My Audible audiobooks show up in iTunes, 'Sync Audiobooks' does not sync them to my connected iPod. Items are selected, computer (Win10) is authorized... enough space on the iPod - what's up?

By the way, over the decades using Apple's Help files, I've found a succinct answer is very difficult to find. Appreciate any input, thanks.