HOW Released is calculated on Purchase order

HI,

I am trying to bring released in report by below query however for some PO's its bringing correct results and for others its showing null when the item is null

SELECT poh.po_header_id,

    SUM ( DECODE (POL.order_type_lookup_code, 'RATE', PLL.amount - NVL(PLL.amount_cancelled, 0), 'FIXED PRICE', PLL.amount - NVL(PLL.amount_cancelled, 0), NVL(PLL.price_override, 0)*(NVL(PLL.quantity, 0) - NVL(PLL.quantity_cancelled, 0)))) AMOUNT_RELEASE

  FROM po_line_locations pll ,

    po_lines pol ,

    po_headers poh

  WHERE 1                                =1

  AND poh.po_header_id                   = pol.po_header_id

  AND pol.po_line_id                     = pll.po_line_id(+)