Some posts cannot be found

At
http://www.taiwaneseamericanhistory.org
a search for 長青 returns seven results, but if I deactivate Relevanssi, I get 16.
My first question is how I can fix this problem of some posts being not found by Relevanssi.

One post that above search does not find is
楊秉彝長青教室影片集 Peter Yang’s Video files .

But I am puzzled by the fact that if I were to search for a longer string 楊秉彝長青教室 which includes 長青, it will find the post.

I appreciate your help. Thank you, Luby

https://wordpress.org/plugins/relevanssi/