Mail for mac does not find e-mails of the last month

Mail for mac on MacOS High Sierra 10.13.6 is not able to find messages received over the last month.

i.e. if I search for messages "from tony bruce" it only shows messages received until the 30th of august, but nothing in September. Alternatively, if there is a message from Tony Bruce in a thread, it goes to the message of the thread of the last month. But never the last one. Doesn't matter if I search for name, address, subject ecc... always the same problem.

I have synched two gmail mailboxes with IMAP configuration, nothing very special, but it's very frustrating and annoying. Any suggestion?

Thanks

If you don’t get expected results when you search for email messages in the Mail app on your Mac, try these suggestions.

1. If a specific mailbox is selected in the Favorites bar, Mail searches only that mailbox. To search all mailboxes instead, click All.

2. Mail may not be looking in the Trash or Junk mailboxes or searching encrypted messages. To include them when searching all mailboxes, choose Mail > Preferences, click General, then select the options.

3. You may need to sync the mailboxes on your Mac with those on the mail server. Choose Mailbox > Synchronize > [account].

4. The Mail folder (located in your home Library folder) may be excluded from Spotlight searching. To check, choose Apple menu > System Preferences, click Spotlight, then click Privacy.

5. This solution can take quite some time because all messages will be deleted from your Mac. Therefore, it’s wise to try the other methods first if your mailbox is particularly large. To rebuild a mailbox, proceed as follows:

   In Mail, select the desired Mailbox from the left-hand panel. From the Mailbox menu, choose Rebuild.