If a sync has previously failed with an error and the error has been resolved you can force the order to be synced again by updating any of the properties on the order:
After clicking edit on the properties area, remove the error text so the field (xero_error, myob_error) is blank and click save. This should force the order to re-sync.
Updating in bulk:
Order properties can be updated in bulk on the orders page:
After selecting Update Properties as per the above screenshot, tick the property you want to update (xero_error, myob_error etc) and add an empty space to the field:
Click submit to update all the selected property on all the chosen orders. Please note this may take a few minutes depending on how many orders were selected.