Segment Criteria Problem

Hi,

I'm segmenting a mailing list geographically to create to segments 'North' and 'South'. I have successfully created the segment 'North by selecting 'ISN'T' across 3 criteria 'Account Name' fields (i.e. isn't John, isn't Ajay, isn't Omar). But when I try to create 'South' by selecting 'IS' across the same 3 criteria fields there appear to be no contacts. I've additionally tried 'contains' but the system can't find any contacts. Can you advise?