Improvement of Japanese keyword search relevancy

A Japanese customer, who uses Custom Japanese for Indexing Language, complains about our Japanese keyword search relevancy of Confluence. What the customer exactly faces is:

A content has a phrase "マイグレーションツール" (migration tool), which is a compound of マイグレーション (migration) and ツール (tool). However, the content cannot be searched by the keyword either "マイグレーション" (migration) or "ツール" (tool).

The customer would like us to improve our Japanese keyword search relevancy.