Just posted a new thread on Lisbon transit options (here). I tried to subscribe to it, but came back and noticed that it had new posts but without notifying me.
I looked at my subscriptions, and saw I was subscribed to a thread about Africa travel (this one). I unsubscribed from that, went back to my original Lisbon transit thread and hit subscribe. I went back to look at my options, and saw that I was subscribed again to the African thread.
Looks like it is selecting the wrong thread to subscribe to!