Audit expense reports with missing imaged receipts is not working as expected

Can anyone please advise how to fix this! We have expense reports that require receipts not being selected for audit when the receipt is missing.
We had thought that with the option Audit expense reports with missing imaged receipts ticked in Audit selection rules that anything that had a missing receipt would be sent to audit, but this is not working. Is there another way of ensuring expense reports with missing receipts are selected for audit?