iExpense - Why would report status display as null in expenses audit form?

Occasionally, we've had a problem whereby an expense gets stuck and when the auditor looks at the expense in the audit form, the report status is null.  The workaround is to have the expense submitter re-submit the expense, then the problem resolves itself and the status displays.  But we'd like to know why this is happening. What would cause the report status to be null?  Has anyone else had this issue?   We are on Oracle EBS 12.1.3.