Here's how to troubleshoot:
1. Check the item record if it has enough quantity available under the location set on the order.
2. Check the form used if Location column is set to show under Items tab.
Note: The system respects the location per line item if the location is exposed as a column on the form. If it is blank per line item, all items will be back ordered. The location in the header would only matter if line item location is hidden on the form.
3. If users prefer to use the Location selected on the main header instead of manually selecting the location per line item, then line item location must be hidden on the form under Items tab. Follow the steps below:
a. In the Sales Order record, hover your mouse to Customize and select Customize Form.
b. Navigate to Screen Fields tab > Columns sub tab > unmark Show checkbox for Location field.
Note: You may also opt to use the Standard Form.