Hi,
Once the Purchase order Released your concern is not possible but cross check how many release codes available in you Purchase order and did you released your Purchase order with all release codes?
If multiple release codes available in your Purchase order and you released with single release code, then you need to release your Purchase order with corresponding other available released codes.
Also check-Did you have Purchase order Release indicators setting as below
ReleaseIndicator--Released---Changeability --Value change % ---Description
B ..................................................................................................................PO waiting for Release (Blocked)
R..............................X..................4 or 6..........................................................PO Release
Regardsa,
Biju K