Cost not getting booked for new activity added in Work center

Hi guys.

     I have a production order PO1. PO1 has one operation with work center WC1 (WC1 has one activity defined-MACHINE MC1).

     PO1 is already released.No confirmation has been carried yet.

     Now I have added one more activity in WC1, say LABOUR LC1.

     If I confirm the operation with work center WC1, cost is not getting booked for the new activity LC1 but the hours are booked.

     But if I remove and add the same work center against the same operation or add new operation with same work center, cost is getting booked for PO1.

     I have nearly 500 production orders already released with work center WC1.

     I don't want to remove and add the same work center in the production orders to book the labour activity LC1 costs.

     How to solve this issue?

     Thanks in advance.