Figure out why credit notes/invoices/payments/contacts are not syncing
Review Bill Syncing errors
Where can I find Quickbooks errors
Where can I find Xero errors
Note: there may be multiples pages of errors; you can find these by using the page numbers and navigation errors.
Errors will only display after the sync has completed and if it failed to bring data over from Syngency.
Initial sync can take up to 24 hours to complete
For a full list of Quickbooks Error codes, and how to troubleshoot them, see here.
Go to Settings (the gear icon)
Click on Bill Syncing
Select either Xero or Quickbooks Online from the subtab
Scroll to the very bottom of the page and check for any Sync failures or relevant error messages.