Diagnosing workflow actions showing as executed but no change on transaction?

I have problems where a workflow action shows as executed but the change isn't made to the transation.

I have a purchase order approval workflow with an 'Approved' State where the only action on 'Entry' is: Set Field ValueApproval Status=Approved

Occasionally as a transaction passes through this state it shows as fully executed but it doesn't change status from 'pending approval' to approved.

Any ideas why this would happen?

thanks

Perry