MERGE INTO - TERADATA updates zero

I am trying to execute below query. When I execute subquery alone (derived table B) , I am getting values. But if i use the same in MERGE, it is just updating zeroes.

MERGE INTO DDEWD03S.T1_PRE_TMP TMP
USING
   (
SEL WH_CUST_NO , A_MX_CNT
FROM
(SELECT WH_CUST_NO, A_M1_CNT AS A_MX_CNT FROM DDEWD03S.T1_BASE_TMP WHERE     A_M1_CNT IS NOT NULL 
 UNION ALL 
SELECT WH_CUST_NO, A_M2_CNT AS A_MX_CNT FROM DDEWD03S.T1_BASE_TMP WHERE     A_M2_CNT IS NOT NULL
 UNION ALL   
 SELECT WH_CUST_NO, A_M3_CNT AS A_MX_CNT FROM DDEWD03S.T1_BASE_TMP WHERE    A_M3_CNT IS NOT NULL 
 UNION ALL  
 SELECT WH_CUST_NO, A_M4_CNT AS A_MX_CNT FROM DDEWD03S.T1_BASE_TMP WHERE A_M4_CNT IS NOT NULL
  UNION ALL  
  SELECT WH_CUST_NO, A_M5_CNT AS A_MX_CNT FROM DDEWD03S.T1_BASE_TMP WHERE    A_M5_CNT IS NOT NULL
   UNION ALL  
SELECT WH_CUST_NO, A_M6_CNT AS A_MX_CNT FROM DDEWD03S.T1_BASE_TMP WHERE   A_M6_CNT IS NOT NULL)A
  )B
 ON  TMP.WH_CUST_NO = B.WH_CUST_NO
 WHEN MATCHED THEN UPDATE 
SET   A_MX_CNT = B.A_MX_CNT

My source and target has same WH_CUST_NO AND it is primary index in both. Can any assist.