Hi,
Are you using MTO scenario for your production orders?
As you said, When we change the dates in order manually, rescheduling is done while saving. But in this case we cant save the orders individually.
I think with the help of ABAP consultant, you need to analyse that how the program behave when you change the dates in order manually, how rescheduling is triggered while saving and why it is not happening when you are doing the same in mass.
Find the root cause with the help of ABAP consultant and do the fine tuning for your program.
Read the below SAP Document. It may help to you,
Mass Processing of Process and Production Orders - Process Orders (PP-PI-POR) - SAP Library
Also do the mass rescheduling by COHV for production orders and apply debugger to check whether it works or not? Take the help of below SCN thread for the same,
Rescheduling of Production Orders using COHV
Regards,
Narresh