Keep the app open and connected to a stable Wi-Fi network
Plug the device into power
Restart the device
Update the app and the operating system
Make sure date and time are set automatically
Check that there is enough free storage
Turn off and on the message sync setting
Sign out and sign back in to the account
Remove and re-add the account if needed
Clear the app cache if the app allows it
Leave the device idle for a while so indexing can complete
Avoid using battery saver or low power mode
Check for account or server sync issues
Rebuild or reindex messages if the app provides that option