Sometimes a project will have a remaining balance, but not technically have any payments due at this time. For example, if you have a Net 30 based project, and it's only 10 days after the event, no payment is technically 'due' at this time.
You can, however, add the 'Remaining Balance' Custom Column to see how much your clients still need to pay on their projects.

Click "Edit Custom Columns" to the top right of your project list. Under Billing, you'll see the Remaining Balance column.
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Once added, you can sort by remaining balance and hover your mouse over the column to see the total remaining balance.