Help with a Merge statement in oracle

Hi,
 Below is the merge statement I was using to Update and insert  data into a table. But the problem is  the conditions  in the Update statement are not working. I mean rows are not being updated, but when I remove one of the condition then it works.

MERGE INTO table  a
     USING table b
        ON (b.id = a.id)
WHEN MATCHED
THEN
   UPDATE SET
      a.col1 = b.col1,
      a.col2 = b.col2,
      a.col3 = 13
           WHERE  a.col3 = 6  and
                      b.col4 ='N'
WHEN NOT MATCHED
THEN
   INSERT     (a.col1, a.col2, a.col3)
       VALUES (b.col1, b.col3, 1)
       where b.col4 = 'N'