Check the following:
Are you online or offline?
Make sure each device with Logos installed is online.
Are you using a firewall?
Misconfigured firewalls can block sync operations.
Can you access the internet on both devices?
If not, address this issue first.
Have there been any recent changes to your network?
Network changes can break connectivity.
Is cellular data enabled for the Mobile app?
If you are using a cellular connection please ensure that the mobile app is allowed to use data.
Did you recently update either the Desktop or Mobile apps?
After an app update it can take some time to sync notes/highlights.
Are you using a new computer or mobile device?
Restoring a backup of Faithlife apps to new devices isn’t supported, which means your Library will have to sync to your new device from the ground up.
Are you signed in to the app?
Notes and highlights won’t sync to anonymous accounts.
Are you using the same account for Desktop and mobile?
Notes and highlights can’t sync between different accounts.
Did you check View Settings to verify that the Notes setting is turned on?
If the Notes setting is turned off, notes and highlights will not be visible within your resources.
How long did you wait for the notes and highlights to sync after signing in?
If you have thousands of notes and highlights in a single document, it can take a long time for them to sync.
Do your notes appear on documents.logos.com?
Check documents.logos.com to see if your notes appear there. If not, there is likely a network problem.
If they are present at documents.logos.com, which device are they missing from? Has that device been synced while online?