1. When importing a segmented house file, the file typically contains 3 fields - a finder number, a source code, and an ID number for the constituent.  The ID can either be the system record ID, the lookup ID, or an alternate ID for the constituent.  Ensure that whether an existing or new layout is used, the ID column specified is one of these three (not the finder number or source code) fields.
  2. Ensure that the ID column is the first column in the import file, so that all constituents can be identified appropriately.