Discount calculation against to Vendor Invoice based on pay Terms

Hi Guru's,

We post invoice from Legacy system to SAP via IDOC .There are no PO/GR reference in SAP.

We have Pay Term maintained as 1 % of 15 days .when we run Payment job we are not seeing discount calculated.

searched may blogs and couldn't find relevant details.May i know how the the control to have Discount calculated based on Pay Term when we do Payment to vendor.

Let me know for details.