Wrong purchase group is being picked in the PO from SC

Hi Experts,

We have an issue where wrong purchase group is being picked in the PO.Normally in the backend the purgroup should be picked as per below logics.

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

2)  Else, the first line item in the SC is taken to determine the purgrp via the item’s product category using a customised table.

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

Please let me know if any clarifaication is required.Thank you for your response