Requisitioning BU have null value for header and header lines

I made the following query to give me the open agreements and their lines,

in the table, some lines had a Requisitioning BU value but the majority does not.

In this query, none of the rows have a value for the req_bu_id

Am I using the wrong tables, or am I should modify the conditions of the query ?

The query:

select pha.po_header_id agreement_id, 

pha.req_bu_id agreement_req_bu_id, 

pha.segment1 contract,  

pos.party_name supplier,

pha.currency_code,   

pla.po_line_id agreement_item_id,  

pla.item_id inventory_item_id,