This is a very good script to find blocking chain with lead blocker. I got this script from Pinal dave blog. Below Script gives you blocking tree where you can easily find out the lead blocker and do further investigation.

SET NOCOUNT ON GO SELECT SPID, BLOCKED, REPLACE (REPLACE (T.TEXT, CHAR(10), ' '), CHAR (13), ' ' ) AS BATCH INTO #T FROM sys.sysprocesses R CROSS APPLY sys.dm_exec_sql_text(R.SQL_HANDLE) T GO WITH BLOCKERS (SPID, BLOCKED, LEVEL, BATCH) AS(SELECT SPID,BLOCKED,CAST (REPLICATE ('0', 4-LEN (CAST (SPID AS VARCHAR))) + CAST (SPID AS VARCHAR) AS VARCHAR (1000)) AS LEVEL,BATCH FROM #T R WHERE (BLOCKED = 0 OR BLOCKED = SPID)AND EXISTS (SELECT * FROM #T R2 WHERE R2.BLOCKED = R.SPID AND R2.BLOCKED <> R2.SPID)UNION ALL SELECT R.SPID,R.BLOCKED,CAST (BLOCKERS.LEVEL + RIGHT (CAST ((1000 + R.SPID) AS VARCHAR (100)), 4) AS VARCHAR (1000)) AS LEVEL,R.BATCH FROM #T AS R INNER JOIN BLOCKERS ON R.BLOCKED = BLOCKERS.SPID WHERE R.BLOCKED > 0 AND R.BLOCKED <> R.SPID) SELECT N' ' + REPLICATE (N'| ', LEN (LEVEL)/4 - 1) +CASE WHEN (LEN(LEVEL)/4 - 1) = 0 THEN 'HEAD - ' ELSE '|------ ' END+ CAST (SPID AS NVARCHAR (10)) + N' ' + BATCH AS BLOCKING_TREE FROM BLOCKERS ORDER BY LEVEL ASC GO DROP TABLE #T GO

After execution of above Script you will get output like this:-