That's an insightful observation, @Cgerman.
It's possible that there are no open invoices for that specific customer. I suggest double-checking to see if you've created an invoice for them. You might have inadvertently created it for a different customer instead. To verify, please follow these steps:
1. Go to My apps and select the Customer Hub.
2. Choose Customers and click on the specific customer you want to verify.
3. In the Transaction List section, filter the transaction Type and Status to view invoices associated with that customer.

Also, you can check the status of the invoice. It could be that the invoice has already been paid, or the customer has an existing credit that was automatically applied to their open invoices. This could explain why, when you receive payment, it doesn't show up.
If you have any follow-up questions, please feel free to comment below.