Not IN() Innaccuracy

I want to view a list of zip codes that are in list1 but not in list2 so that I can see which area was hit with which mailing (1st or 2nd). I have tried the below query but it is returning 0 records, and after a manual line by line I found at least 80 zips that were in list1 but not in list2. What is the issue with my query that caused the zips to not be returned as not in()

Select marketingtype, zip As [Area Marketed]
From campaignDB.list1
WHERE marketingtype Is Not Null
AND zip NOT IN (Select zip from campaignDB.list2)
Order By zip ASC