Something has to be blocking the truncate table statement. This is kind of an old school script as it uses code from sp_lock, but it should tell you exactly which connections have locks on the table and what type of locks they are.

declare @tablename sysname

select @tablename = 'Replace with your table name'

select convert (smallint, req_spid) As spid,

rsc_dbid As dbid,

rsc_objid As ObjId,

rsc_indid As IndId,

substring (v.name, 1, 4) As Type,

substring (rsc_text, 1, 32) as Resource,

substring (u.name, 1, 8) As Mode,

substring (x.name, 1, 5) As Status

from master.dbo.syslockinfo,

master.dbo.spt_values v,

master.dbo.spt_values x,

master.dbo.spt_values u

where master.dbo.syslockinfo.rsc_type = v.number

and v.type = 'LR'

and master.dbo.syslockinfo.req_status = x.number

and x.type = 'LS'

and master.dbo.syslockinfo.req_mode + 1 = u.number

and u.type = 'L'

and rsc_objid = object_id(@tablename)