PO on multiple vendors on the basis of service-sub line items

I have a requirement

My PR has only one Service line item with different service sub line item. I want to place order on different vendors for different service sub line item. While placing first order i deleted certain sub-service line items which are not required and then placed order for the required.When i create the second PO referring the same PR, i get a message service item ordered in full.

Can anybody help me on this or is there any notes on this?

Jayaraman