Have you double checked to make sure these Audiobooks are configured to sync to your iPod from under the iPod's Books configuration tab in iTunes?
To access this configuration tab, plug in your iPod and select it from under the Devices section in the left hand pane of iTunes to bring yourself to the iPod's Summary tab. Now locate the Books tab and select it. Here you can configure what and how audiobooks are synced to the device.
If you make any changes to this tab, make sure to hit the Apply button on in the lower right hand corner.