Is there any padding on either end of the lookup? Try doing a trim on your source data and then trimming your lookup column as well. Unless you go into the advanced settings of the lookup it is doing the comparison in .NET not SQL so trailing spaces/blanks will have an affect.
Jack Corbett
Consultant - Straight Path Solutions
Check out these links on how to get faster and more accurate answers:
Forum Etiquette: How to post data/code on a forum to get the best help
Need an Answer? Actually, No ... You Need a Question