Audiobooks is a separate category of syncing settings in iTunes ( https://support.apple.com/en-us/HT201253). Are you already specifying items to sync in this way?

Additionally, you can drag audiobooks in your library directly to your iPhone in the sidebar.

For me, I found the iTunes syncing cumbersome. I use the app called Bookmobile and find it so much more simpler and efficient to manage audiobooks.