If some data are not synced to Xero it is possible to synchronise invoices and payments over past dates.
The synchronisation can be done by
Go to Setup > Connections > Integrations > Configure Xero.
Click on the Xero account to access the setup page.
Click on the Tools tab.
Select the Dates From and To.
Click Enqueue Request.
It can take a few hours for the data to be synced.