the "find" tool does not work if seraching for a sender or receiver

Wherever I digit John Brown or Jim White - whether in find tool or in little window on top of page - I don’t get all the mail exchanged with John Brown or Jim White-

Hi, I am able to use search with your scenario without any problem, can you tell me what email service and protocol (and if it is IMAP if you have fully synchronized emails) do you use?