BAPI_PR_CREATE is not filling "Fixed Vendor" Field

Dear Experts,

We created a program for uploading Excel to SAP for easy creation of purchase orders. The program was working very well until two days ago, but now it doesn't fill "Fixed Vendor" field.

When we (manually by ME51N) create PR "Fixed Vendor" field is filled automatically for the same material.

Could you please help?

PS: We are not filling "Fixed Vendor" field by BAPI, we want it to fill automatically.