There are two potential reasons for this:
1. Custom Transaction Numbers for your QuickBooks invoices are not supported through QuickBook's API. When this setting is enabled in your QuickBooks 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 QuickBooks' system-generated invoice numbers when using Ignition and QuickBooks together. The system-generated invoice number is simply a sequential numbering system.
Please note that disabling Custom Transaction Numbers should not affect QuickBooks' ability to generate invoice numbers, either manually or automatically (via Ignition).
Turning off custom invoice numbers in QuickBooks allows the Ignition -> QuickBooks integration to work properly and raise invoices automatically whether it’s for a one-time or recurring billing type - without affecting your ability to raise invoices manually. QuickBooks will use the system-generated invoice numbers to assign invoice numbers automatically.
If you are not sure if you are using custom transaction numbers:
Log into your QuickBooks 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: