In a Relationship based lookup, the matching data has to be exact. Partially matching data in the match fields aren't going to match records.
You may find the search portals at the bottom of the layout in this demo file of interest: https://www.dropbox.com/s/0pm1gdqcfi2ndpv/EnhancedValueSelection.fp7
The search portals illustrate different forms of partial data matching through the use of a script supported portal filter. FileMaker 12 users can open this file from the File menu to get a copy converted to the .FMP12 file format.