This a very long-standing problem which I suffered from for about 12-18 months. LT was just broken for me. Apple don't seem to have acknowledged the problem publicly.
See this thread for a discussion of this problem.
http://discussions.apple.com/thread.jspa?threadID=1392289&tstart=0
Lots of possible solutions there - none of which worked for me. You might be more fortunate.
However, having just updated to ProKit 5 and then FCS3 (from FCP 4.5), LT now works again for me. I don't know whether it was ProKit 5 or FCS that solved it however.
Good luck.