Missing mails when using search function

Hello all,
I have done some research about missing emails issue when using the search function, but it does not solve the problem.
Here is what I discovered:

  1. When searching the email folder with a keyword in “Subject” (the keyword was contained in the mail’s subject), the mail could not be found.
  2. When I searched that mail with its full subject, the mail was found. If I searched with this keyword in “Subject, sender, recipients, body, notes and attachments”, it was pulled out. Actually, the mail could only be found by this searching option. All other options with narrower searching fields could not find that mail.
    I think it is a bug, rather than an improper operation of eM Client.
    Any suggestion? Thanks