Try these solutions below. After trying each step, synchronise again before moving to the next step to see if it works.
- Make sure your internet connection. Weak or unstable internet connection may cause synchronization issues as well. Check if your OneNote is up-to-date with the latest version and update it if it's not.
- Restart OneNote and your iPad to clear any temporary glitches or conflicts that might be affecting the synchronisation process.
- Clear OneNote caches and confirm that you have the necessary permissions to access and sync the OneNote while you are there.
- Check if your iPad still has enough storage space. Running low on storage can impact the performance of your apps, including synchronisation so backup and or delete unnecessary files and apps if you are low on storage space. Check on your OneDrive storage as well.
- Remove and re-add your account then try synchronising again.
- Uninstall and reinstall OneNote as last resort if the above 5 steps all failed.
If the issue persists, try using a different device (preferably an iPad like yours) to access your OneNote notebooks to see if the synchronisation issue isn't specific to your iPad.