Search for group members doesn't show some members

Some of our members seem invisible to the Search function. They don't show up when searching either their first or last names, even though other people with the name show up in the resulting list.

A search for Michael shows 6 profiles, but there are 7 members named Michael. And a search for the missing Michael based on his last name does not show him.

Likewise, a search for Beth does not return the only member with that name, but does return an Elizabeth.

Apparently there are other members missing from the search too, based on others' reports.

Any ideas?

Thanks.

Bill