Execute sp_who3 (internal tool) for a blocking scenario.
Collect PSSDiag on the SQL Server computer. This typically requires contacting Microsoft Customer Service and Support (CSS) because of the configuration options in PSSDiag that are needed. There are many times when it has been configured, but with incorrect settings. As a result, the data might have to be re-captured.
Investigate orphaned DTC transactions, which show up as -2 SPID.