Preemption checking requires searching beyond the world of Westlaw and Lexis because neither has exhaustive coverage - some full-text coverage goes back to only the 1990s. Look at the resources for preemption checking--it includes other databases you should consult before spending time researching and writing your case note or comment.
If you find a paper on your topic that appears to preempt you, it may just be a matter of revising your thesis to avoid preemption.