Functional help in Work Center Group and Work center

I am working in AX2009, I have assigned work center group to each operation in Route and each work center group has 5 work centers so when I schedule a production orders, every production order is reserved to 1 work center remaining 4 work center is not reserved at all. 

I thought all work centers will be equally reserved with different production orders but it is not.

Please advise me to find a way to equally reserve work centers for various production orders.