Bibliographic records which do not overlay s.l. records are called non-overlays. Most common reasons:
Print each record from Create Lists & remove from the list as resolved. Search for all potential records: s.l. records, paper records, MF records, serial/monographic records.
Searching and editing will need to be done in Catalog mode, not Create Lists mode. Very occasionally, text-based Mondo will be more appropriate than Millennium.