SQL query not giving proper result

I have a query with multiple joins for which DOC_TYPE column is coming NULL even if it has some values in it. The query is below

SELECT a.mkey,
       c.type_desc                      DOC_TYPE,
       a.doc_no                         INWARD_NO,
       CONVERT(VARCHAR, a.doc_date, 103)date,
       a.to_user,
       a.No_of_pages,
       Ref_No,
       c.type_desc                      DEPT_RECEIVED,
       c.type_desc                      EMP_RECEIVED,
       b.first_name + ' ' + b.last_name NAME,
       b.email
FROM   inward_doc_tracking_hdr a
       LEFT JOIN user_mst b
              ON a.to_user = b.mkey
       LEFT JOIN type_mst_a c
              ON a.doc_type = c.master_mkey
                 AND a.dept_received = c.Master_mkey
                 AND a.emp_received = c.Master_mkey
WHERE  a.to_user = '1279' 

The doc_type value is 428 and whose desciption comes from

select type_desc from type_mst_a where master_mkey = 428

as Drawing but when I run the join query I get it as NULL. why ??

I am using sql server 2005