Symptoms:
The "Check Interval" wasn't active. There was no automatic checking for new emails, or changing BOLD type for email accounts back to regular type after reading the new emails, or updating the number of unread emails.
Clicking on "Check Now" from the X-notifier pull-down menu was a manual work-around.
The fix is to click "Enable" from the X-notifier pull-down menu, which is too easy, and it removes the big Red "X" from the top of the X-notifier icon, and re-enables the Check Interval.
Why didn't I know that? Thanks, CyberTooth.