There are two potential reasons for this: 

1. Custom Transaction Numbers for your QuickBooks Online (QBO) invoices are not supported through QBO's API. When this setting is enabled in your QBO account, invoices will still be generated but you will not see an invoice number either in your sales>invoice grid view, or on the invoice itself. 


You will need to rely on QBO's system generated invoice numbers when using Practice Ignition and QBO together. The system generated invoice number is simply a sequential numbering system.

If you are not sure if you are using custom transaction numbers:

 - Log into your Quickbooks online account  
 - Navigate to settings>company settings>sales
 - Here you will see if Custom Transaction Numbers are enabled:

2. If you are not using Custom Transaction Numbers, but an invoice number is not displaying on your invoices, this is in fact a known issue to Intuit, which appears to have been resolved. If you are still experiencing this issue, follow the instructions within the comments of this Intuit Community thread:

https://community.intuit.com/questions/1647518-when-i-go-to-print-or-preview-an-invoice-the-invoice-number-is-missing-how-do-i-get-it-to-appear

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