Discount available on voucher match?

I'm having issues whereby,  the discount will not calculate if I'm doing a voucher match with a PO.

I have a payment term default on a supplier (in the address book).  ie 2% 10/net 30.

The PO has been processed, the voucher matched (which the correct payment term code in the grid).

But when creating payment group, the discount does not appear.

If I enter the same supplier, (same default payment terms as above) as a Standard Voucher, the discount works correctly.

Is there a processing option either on the Purchasing side or the AP side that I'm missing?

Any thoughts would be greatly appreciated.