Vend cash discount info is missing

Hi all

I am very new to the Ax 2009.

currently I am using  "LedgerPurchaseInvoiceService" to push invoices to AX through AIF.

I know this service will generate record for table LedgerJournalTable,LedgerJournalTrans, VendTransCashDisc and  VendInvoiceInfo.

The problem is the vend cash discount info is missing in VendTransCashDisc table. but it actually is avaliable in LedgerJournalTrans table.

can anybody tell me how is the vend cash discount info be created in  VendTransCashDisc by LedgerPurchaseInvoiceService

Thanks a lot

Phil