Sometimes there may be an error with your QuickBooks Online Sync.
To see exactly what the error is, you will have to view the Sync Log.
This article describes how to do that.
1) Press the View Log button.
2) The top part of the log shows all the syncs you have performed and whether or not they are successful.
Click on the link for the sync that you want to look at.
3) The bottom section now shows the details from the sync. Scroll until you get to the red error message.
Resolution for the error will depend on what the error message is. In this example, an invoice already exists in QuickBooks Online that has the same invoice number as an invoice in invoiceASAP. The resolution is to change the invoice number in either invoiceASAP or in your QuickBooks.