Smart Mailbox Not Updating

I have two simple smart mailboxes. One (red flag) to highlight business mail I need to deal with, and one (green flag) to highlight personal mail I need to deal with. The system works well most of the time, but recently I have noticed that sometimes when I clear a flag, the messages stay in the smart mailbox. I am checking the mail on two different computers, and the flag updates in the mailbox on both, but some messages stay in the smart mailbox after I clear the flag. Both computers are running HS 10.13.6. Any clue how to address this? Seems to be happening only in my personal mail (gmail) account.