Correct me if i'm wrong, you could also use the below script to trace back to what's being blocked by whom.

select * from sysprocesses

order by blocked desc

Then run the below script to view what the actual blocking spid is doing. (replace 87 with the real blocking spid)

select * from sysprocesses p

cross apply sys.dm_exec_sql_text(sql_handle)

where p.spid = 87

then if needed you could kill the blocking spid if the proccess can be safely removed.

kill 87

This would also work when you have issues with activity monitor loading... or when you're unlucky enough to be in an environment where you cannot add sp_WhoIsActive.