I appreciate you for sharing detailed information. It seems that the sub-customer was set to Bill with parent, which is why the statement shows a zero balance.
To correct this, let's go to your customer information page and switch settings to Bill this customer.
Here's how:
1. Hover your mouse on the Sales tab at the right panel. 2. Select Customers. 3. Click on the sub-customer's name. 4. Click the Edit button at the top right.
Under Is sub-customer, one of the drop-down boxes was named for a specific customer. Click on the 2nd box and choose Bill this customer. Then, click Save.
That should let you create and print a statement showing your sub-customer's overdue balance.
For future reference about creating, managing, editing and customizing your statements, you can also check this article link: Create and manage statements.
Feel free to post here in Community if you have other concerns. Cheers!