HOw to Derelease a PO through Me28

Hi Experts ,

I made a one with (Tcode Me21n) Purchased order and Release this PO with Tcode ME28 now i m not able to De Release The Same Po with Tcode ME28 . i m going to De Release The PO that time i got a Message

Release prerequisite not fulfilled

Message no. ME102

Diagnosis

A prerequisite for release using the code you entered is prior release

by other codes.

Procedure

Find out the order of the release codes in the current release strategy.

This is Happaning with only few PO . Pls suggest me why this happaing and soulation .

Thanks

Prodipto