The Key to finding out why your invoice isn't syncing is to check the Event Log.
Checking the Event Log:
Go to Reports > Business Reports > Log Reports > Event Report
This will show you a log of all the errors for your invoices. This would be the FIRST thing I would do if your invoices are not showing up. In the case of this article, you will find the following error:
Click on image to enlarge
The key parts you need to be looking for are the following:
1) The invoice number in the top right hand corner: In the example above the invoice number is 641. This defines which invoice the error is talking about. If you are looking for a specific invoice, you will find it her.
2) The error message: This error message will let you know what the issue is.
3) The Reason For the Error: The highlighted areas in the screenshot above are where you will find this specific information. For guides on the specific errors, please go to the articles below:
Invalid status for modification
If there is no article for the error, please email firstname.lastname@example.org