There is a strong correlation between the information entered during the listing input process and the results found when users search the system.
If a field is left blank during listing input, Matrix interprets that field as "unknown". On the search side, if a user includes that field in a search, Matrix excludes the listing from the search results, since it has no selection or value.
WHAT TO DO:
More is better in this case! When entering a listing, include as much information as possible, even the fields that aren't required. The more fields you leave blank, the more you risk your listing not being included in search results.