lock timeout

I have a problem related to sql server performance.
I'm managing a big DB with many stored procedured and users connecting to it. Some times it is very slow. I was checking the blocked processes using sp_who and no process was blocked but in the same time there were many lock:timeout events in the profiler!!!! what is the reason? how can I find which query/stored procedure is causing the lock timeout and why there was no blocked process in sysProcesses??
Any help is appreciated.
Thanks

Hi Are you running sql 2005?, If so try setting the Blocked process threshold to a certain value, and try capturing the queries in profiler? Did you find any queries that were running for a long time ? You can also try using this:http://support.microsoft.com/kb/271509
Thanks
Jay