This failure of the Notification system happens with some frequency on Genforum.
In the past, I have found the only way of drawing attention to it, was to make a "false" posting, then use the "Notify Administrator about this message? " at the foot, as otherwise there did not appear to be a way of contacting directly.
That was followed by a few postings back and forth to re-establish the Notification system.
I have also recently had such Notifications come through in groups, several days after the Follow-Ups had been posted and seen "o-forum" by me.
Hence it is worth checking the relevant forums each day.