Hi,
Your session could be hanging if its waiting for some resource,currently used by other sessions or process.Try quering v$system_event,v$session_event and finally v$session_wait to get an idea of the events waiting for the resource.Probabaly ur session might be experienceing "buffer busy waits",if ur triggering is doing some DML activites or having some I/O related problems.

regards
anandkl