...and you've checked that they are subscribed in the forum in question? Being enrolled in the course is only part of the story.
Yes, they are enrolled and subscribed. Still they don't receive any forum notifications. what could be the problem?
Have you checked the email logs on the server? That will certainly give you some clues about what is happening.
This forum post may also give you some ideas: https://moodle.org/mod/forum/discuss.php?d=386816#p1559438
This forum post may also give you some ideas: https://moodle.org/mod/forum/discuss.php?d=386816#p1559438
we have checked and still there are no notifications to be sent at all! Are there any Moodle Default settings for the forum?