Credit memos not getting adjusted against teh standard invoices after Auto Select in PPR completes.

I have a vendor which has some active credit memos sitting due to be paid along with Standard invoices not enough to pay the vendor. So it is in Credit Balance. However when I process the payment , it ignores the unpaid Creditmemos sitting out there and build the postive Standard Invoices and trys to pay them. Why would it happen when the CM should 1st get adjusted against the standard Invoices and if it is really a positive amount , then it should get paid ? Right.