Wrong purchase group is being picked in the PO from SC

Hi Experts,

We have an issue where wrong pur group is being picked in the PO. Normally in the back end PO pur grp should be picked as per below logics.

1. If a buyer has approved the SC, the pur grp is determined bu buyers SRM attributes.

2.Else, the first line item in the SC is taken to determine the pur grp via the item's product category using a customized table.

we are following classic scenario and our portal version is 5.0.

Please let me know if any clarification is required, thanks for your response.