I recently had a similar problem, and I'm on the InVision servers.
In my case I personally had a day where I got no email notifications, even though I'm following everything and people were actually posting. I saw that through Unread Content, so was able to stay involved in the discussions. Sometime late in the day I mysteriously started getting the notifications, and it hasn't stopped again since. But I never did get the notifications I'd missed. Just started getting the current notifications.
I had checked to see if I am following those topics, and was. And I have checked to make sure it hadn't gone to spam or anywhere else.