The Unbilled Job Report in Accounts Receivable Billing is designed to report Jobs that are unbilled. The problem with the report is that it requires a link from the Job Cost Task Codes to the Job Billing Line Items (which is not always a reasonable approach).
A better method of determining your ‘unbilled’ status on a Job is to create a Job Cost Formatted Report with the Date Last Billed, Contract Amount, Amount Billed, and a field called Amount Unbilled that is calculated by subtracting the Amount Billed from the Contract Amount field.
Then a filter that shows Job where the Amount Unbilled is not equal to zero will provide a useful report.