For the followers topic it doesn’t matter if the stream is live or not.
If you’re not getting notifications it means either your subscription has expired/wasn’t created (you can check this by using the endpoint I previously mentioned), or the channel you’re testing follows on differs from that of the topic, or notifications are being sent but either your route or your handler isn’t correctly handling the POST requests.
Also keep in mind that webhooks rely on the underlying API, so there is a delay due to caching and waiting for all servers to reach a consistent state. One thing you can do is send a request directly to the topic, as a normal API request, if you see that there’s a change in followers there, then a notification should be sent out shortly after.