All Fixed Fee type work items will appear under Unbilled, unless you've created and approved an invoice with invoice type “Final”.
However, Time and Material fee type work items will only appear in the Unbilled screen when:
The work item does not have an approved invoice with invoice type “Final”
Or has unbilled time entries or expenses
If your work item is set to 'Non-billable' it will not show up in the Billing section.