Displays the last statement sent from a client to Microsoft® SQL Server™.
DBCC INPUTBUFFER (spid)
You can use this command to see what is the longest running command which may point to the transaction that is causing the blocking.
Displays information about the oldest active transaction and the oldest distributed and nondistributed replicated transactions, if any, within the specified database. Results are displayed only if there is an active transaction or if the database contains replication information. An informational message is displayed if there are no active transactions.