Salutations Sherry78,
If these audiobooks were purchased from the Book app, downloading them on to your iPhone may be an option - Redownload books and audiobooks
How to redownload books or audiobooks on iPhone
1. Open the Books app, then tap Reading Now at the bottom of the screen.
2. Tap the Sign In button or your photo in the top-right corner of the screen.
3. Under My Purchases, tap either Books or Audiobooks.
4. If you use Family Sharing, you can tap a family member's name to see their purchases.
5. Tap "Not on this [device]," then tap All Books or All Audiobooks.
6.Find the book or audiobook that you want to download, then tap the Download button.
When syncing an audiobook that came from another source, be sure that in addition to checking the Books app, you also check the Music app on your device. If the file did not originate from Apple, it may be seen as music and show up there instead.
You can also remove the synced item and attempt to sync it once more - Sync audiobooks between your Mac and iPhone, iPad, or iPod touch
To remove an audiobook from just your device while keeping it on your Mac, follow these steps:
1. Connect your device to your Mac.
You can connect your device using a USB or USB-C cable or using a Wi-Fi connection. See Sync content between your Mac and iPhone, iPad, or iPod touch over Wi-Fi.
2. In the Finder on your Mac, select your device in the sidebar, then click Audiobooks in the button bar.
3. In the Audiobook list, deselect the checkbox of the audiobooks you want removed.
4. Sync your Mac with the device.
Thanks.