Sounds like your reference to the customers table is broken. Did you copy the tables into one file or are they still external files?
Try re-establishing your relationship and check your exteral data sources and make sure the path is correct and there is only one path the customer table.
The two files are external files. The relationship path seems good and does indeed work for 1 or more new records in the second file. Then it just seems to quit looking up.
thanks for your time, john