Hi Ganesh
This is probably a rescheduling issue. If the planned order dates are changed to after the requirement date, they must be included on the rescheduling check on customizing and within the rescheduling horizon.
Check the following WIKI for more details about the rescheduling check and the relevant customizing settings:
http://wiki.scn.sap.com/wiki/display/ERPMan/What+is+rescheduling+and+how+does+it+work
BR
Caetano