SQL Statement not seeing null values

My statement below gives me what I need EXCEPT the user_def_fld_2  has both null and text values, I need to see both, but currently only see results with text values.

I am new to SQL

SELECT HDR.ord_no AS 'Order#',HDR.slspsn_no AS 'Slsmn#',ord_dt AS 'Order Date',HDR.cus_no AS 'Acct#',HDR.ship_instruction_1 AS 'Ship Note',bill_to_name AS 'Bill To',Ship_to_name AS 'Ship To',hdr.user_def_fld_2 AS 'Header Note',LIN.item_no AS 'Part#',LIN.item_desc_1 AS 'Description',LIN.qty_ordered AS 'Ordered',LOC.qty_on_hand AS 'On Hand',(LOC.qty_on_hand-LIN.qty_ordered)AS 'Balance'
FROM oeordhdr_sql HDR
INNER JOIN oeordlin_sql LIN
ON HDR.ord_no=LIN.ord_no
LEFT OUTER JOIN iminvloc_sql LOC
ON LIN.item_no=LOC.item_no
WHERE HDR.user_def_fld_2 NOT LIKE 'SHOW%' AND HDR.bill_to_name not like 'REGO%' AND HDR.bill_to_name not like 'RFTC%' AND HDR.user_def_fld_2 NOT LIKE '30 DA%' AND HDR.ord_type='O'
ORDER BY HDR.ord_dt ASC