Hi Krishna,
How are PO's being released in your system? Is there workflow, is there auto approval until a certain amount and above that amount it has to be approved by a different approver?
Ideally Released by & Released on fields should get updated immediately when PO is Released.
I suggest you check all the scenarios that would Release the PO.
Maybe also compare the PO's that have these fields updated and the PO's that have these fields blank.
Please let us know what you find.
Cheers.
Shiv.