Auto Create Purchase Order does not default supplier site

Hi,

   I haveused auto create purchase order on one of the requisition for specific vendor A(e.g. vendor1) and it automatically defaulted its supplier/vendor sit. But when i have tried again the auto create purchase order to another vendor/supplier, it does no default it supplier/vendor site.

   I have checked the set up on the payables and purchasing checkbox was ticked for supplier site which did not defaulted the value.

   Can you please provide to me on how to set the supplier site automaticatly when autocreate Purchase Order was used. We want the supplier site to automatically be populated and not be manually chosen.