Lookup fields

[LJH]

I inherited a database a short time ago. Before this I had not used Filemaker Pro at all. I got all the books and have read large parts of them. The main database has a related database called "states" where I have assigned several fields to look up values (dues, subscription type, region and state name). I'd like it to lookup from values in one of two fields (State Abbreviation OR Country). There is never an instance where both fields are used. Both fields are "linked" in both tables, both fields are in both tables. I've got it to work for both fields but not at the same time. (For example, if I tweak it just so that it works for state abbreviation in one record, then it won't lookup when I enter a name of a country in the next record.) What am I doing wrong? Thanks for your help.