In the standard report "Purchase order history" we see that for few Purchase order items the respective Purchase request Id is not appearing.
SAP Business ByDesign
currently there a limitation that this report "Purchase order history" does not display Purchase request ID if the respective purchase order item has any follow up documents.
You can proceed with using another Data Source "Purchase Order Delivery Reliability" or building a report on top of the data source to have these details.
Purchase request item ID , KBA , SRD-SRM-PRO , Purchase Requests and Orders , AP-PRP , Purchase Request Processing , Problem