Hello
This issue is generally due to the rescheduling settings. Either the PO is without the rescheduling horizon or it is not included on the rescheduling check in customizing.
Please review the following WIKI for more details:
https://wiki.scn.sap.com/wiki/display/ERPMan/What+is+rescheduling+and+how+does+it+work
Best regards
Caetano Almeida