If you wanted to subscribe to a topic on the old forum, when you had made a reply to the topic, there used to be a setting where you could automatically subscribe to any topic you made a reply to.
In this current forum version, it appears as if you are automatically subscribed to a topic after replying (the bell/+ icon becomes a bell/tick icon) but when you move away from the topic the subscription has not actually taken place and you have to go back to the topic and subscribe manually.
I also found I had to manually whitelist the VM email address for notifications to stop my email service from totally blocking the notifications as spam, resulting in non-delivery to me.
It's certainly not a very polished notification system compared to the previous version.