iPod classic not syncing audiobook on latest iTunes

When my iPod classic (80GB) is connected to my laptop and I select it, it shows the audio books under "settings" but the audiobooks category in "on my device" shows blank. I've tried automatic and manual syncing. They never show up on my iPod. Prior to this I was able to sync fine. I did relocate my iTunes library to a different external disk, but only the iPod classic is not syncing. What can I do?