Thanks for your reply, Helen! I have double-checked and all the non-editing teachers have "On" for their email preferences.
We had an issue in early May in which the non-editing teachers weren't receiving forum notifications and I think it may have had to do with a Moodle upgrade that was interfering with the notifications. Eventually, it started working. I wonder if it's the same problem this time around.
A few people have had that issue according to this post: https://moodle.org/mod/forum/discuss.php?d=405159.