Prepayment in A/P not be chosen in Create Payment Groups

JDE 9.1 - Created a voucher and paid it using the Prepayment option.  This did create a prepayment voucher in A/P (credit amount).  Now that the open vouchers need to be paid, the prepayment was taken off of HOLD, Create Payment Groups was run, but the prepayment was not processed with the open vouchers.  I thought that the Automatic Payment Process recognizes prepayments just like credits.